Hi guys!
My name is Layven, I play on the EU West server. I'm from Hungary, this might cause a bit raw English, sorry for that. I made this guide about one of my favourite character, Riven. I already played LoL when she was released, so I saw and felt all of her buffs and nerfs. Currently, I think she's pretty strong, so if you don't commit unnecessary sacrifices, she's a big threat. Play safe, be legendary! Take a closer look on my guide.

I decided to start with the skills, to show her possibilities ingame, why is she so strong.
The first thing to notice, that she's a nocost character, which means that she's only limited by her cooldowns, that makes her a really tough opponent.
The second thing, is that she hasn't got only 4 abilities what scales with AD, her passive also get bonus damage from attack damage.

Runic Blade is her passive, granting the auto-attacks bonus 5 / 7 / 9 / 11 / 13 / 15 (+0.5 per bonus attack damage) damage after using skills, but that's not a problem for a nocost fighter with low cooldowns, you just have to time your skills. This passive means, if you have a Bloodthirster with +100 AD, your attacks deal over +150 damage. Pretty cool, isn't it?

Broken Wings is just an epic skill. This grants you full availability to your passive, you don't have to worry about those stacks. Also gives you high mobility, burst damage, high farming potential, and an AoE knockback, a pretty hard CC, because it can interrupt channeled spells like Warwick's or Nunu's ult. After the last patch, which strenghtened this ability, and nerfed the Ki Burst, it's better to max Q first, meanwhile taking a few points on the W.
In addition, this ability has 0.7 AD ratio, so after casted 3 times succesfully, your damage output was 90 / 165 / 240 / 315 / 390 (+2.1 per bonus attack damage) with 1 skill. The greatest AD ratio officially is 2.0 on Caitlyn's and Lee Sin's ultimate, and this is not even ultimate. Interesting.

Ki Burst makes you really annoying, granting you a burst damage with 1.0 AD rate combined with stun, also giving one more stack on your passive. Meanwhile the enemy's in stun, you can hit him to toggle that damage, and easily get away.

Valor makes you a good trader. You can jump to your enemies, getting shield with 1.0 AD rate (WHAT?!), or after you got there with the jumps of your Q, knocked up, stunned him with W, you can leave him with Valor, giving your enemy no chance to react.
After the Q is maxed, I max Valor the second time. The shield is needed in teamfights, the Ki Burst is enough on level 3 to farm easily.
Against hard top laners like Olaf or Renekton, maxing this ability first is recommended. In this case, the best is this:
Blade of the Exile remains active for 15 seconds, gives you higher range to your attacks, and your damaging skills have higher area of effect. It's nice, makes you better in teamfights. Wait.. Oh my god..What the hell? You get +20% Attack Damage. Do you guys know what does it mean? While you are in ult, it seems like you have 1.2 AD ratio on your stun and your shield, and your 3 Broken wings hits with 2.1*1.2= 2,52 AD rate. That's greater than the highest ratio ingame, which belongs to Nunu's Absolute Zero.
But hey, that wasn't enough! You have Wind Slash to shoot, which deals more damage on enemies with low health. You hit'em hard, and even if they flash away, you can kill them with this executioner ability. On full HP enemies, it hits only 80 / 120 / 160 (+0.6 per bonus attack damage), but to enemies below 25% health take 3 times the damage, 240 / 360 / 480 (+1.8 per bonus attack damage). 1.8 Rate on his ultimate.. And of course, that +20% Attack Damage works on this Wind Slash too, the real ratio is 2,16. Unbelievable.

For masteries, I recommend you a 21/9/0 mastery, as you can see. We need as much AD as we can, so take Brute Force and Summoner's Wrath to enhance the ignite. After that, the armor penetration and AD per level makes sense to take. To continue the offence, 1.5% damage and 3% life steal wouldn't be as good as the 4% cooldown reduction. Remember, nocost champions are only limited by the cooldowns. (If you're going to buy Atma's and/or Trinity force, you can take the 10% Critical Damage mastery instead of 1% CDR.) Now you can take Sunder with 6 points of armor penetration, it's always great.
And Executioner has the last point, which really describes Riven's true being. It passes to the Blade of the Exile so much, another +6% to this last-hitting ultimate. This gives her ultimate about 2.3 Ratio below 25% health, but it is still above 2 with 40% hp. You can just execute them.
For the defence masteries, take a point of armor, a point of magic resist, and 2 of Tough Skin . This seems strange, but I can elucidate why is that. On the lane, you trade with Riven the way that you jump on the enemy with knockback, autoattack, stun, autoattack and retreat. Meanwhile, all the minions shoot you. On early levels, they have a really high damage. Your opponent doesn't have a chance to hit you, but the minions damage you constantly. If you watch the Riven Champion Spotlight, Phreak's problem was also this with Singed, that the minions actually damaged nearly as much as he did to Singed. Later he killed Singed,because Riven had incredibly high HP regeneration that time, later Riot nerfed it a little bit, but it would have been even more easier with these 2 mastery points.
After, health/level and health masteries are always great to increase your toughness in teamfights and on lane.

• In my opinion ignite is great for Riven, it helps so much, you can get kills on lane, easily burst down carries in fights, and they can't get away after surviving your ult with 20hp. I use ignite almost every time.
  
• Flash is also very useful.Riven has a lot of dash abilites, but she cant get over the walls. Flash is important, to secure kills, initiate fights or to survive dangerous situations. I use it every time, it can't be replaced.
  
• Actually, Riven takes CC very bad, she's tanky only because of the shield and the lifesteal.Cleanse might be an option, if you see the enemy team has real hard CC, so many that the Mercury won't be enough, you can take it, it's a good choice.
  
• Heal became relatively good after the buff, it can be useful against hard top lanes.( Olaf or maybe Renekton)
  
• It can be a good pick, if you gank botlane with this, or teleport to dragon to secure it. Just ask them to put down a ward. WARNING! Don't use it just to get back to the lane faster. Riven's nocost, has 4 dash abilities, it's really unnecessary. If you take teleport, gank frequently, Riven's got a high ganking potential.
  
• Well, I'm not really an exhaust fan, but its definiatly a strong summoner spell, you can use it with Riven too, it's great to kill the AD carries in the teamfight or surviving ganks.

For runes, I recommend these mixed AD marks. AD/level marks give you the amount of what the flat ADs give after reaching level 8. This 6 - 3 division gives you early game power and farming potential as well as mid and late game strenght. For the seals and glyphs flat armor and magic resist/level is great, armor is for lane durability, magic resist helps you survive a lot of strong AoE spells, which is only needed at teamfights, and there's no need of flat MR, teamfights are usually above the level 9. Against AP top, flat magic resist runes' usage is a considerable option. For the Quintessences, I like to have flat HP, makes me stronger earlier levels, and also you don't need so much HP items, a Frozen Mallet's just enough.

First, I want to let you know, that this item build is just a recommendance. Instead of telling you about this order, I'll just talk about the items. The important thing is to be creative, and buy what is the best in the current situation. The order I have made is just a typical toplane against an AD fighter. I'll list you some items, what works very well.

• Mercury's Treads is one of your most important item for you.It gives magic resist, to survive AoE casters' damage(e.g. Karthus),and it gives you the tenacity. I really love these boots, I buy on almost every solo top champions. Especially, Riven needs this. Riven's weak against hard CC, you just cant get your hits on your enemy, you have no life steal, you cant use your passive properly, neither the stun. If you have this kind of enemy, pick Irelia, or take Cleanse. For a mediocore CC, Mercury's just enough, and required. If they have low AP damage(e.g. Soraka or Lulu mid), of course you can buy Ninja Tabi.
  
• Your core item, Bloodthirster is amazing. You should have it in every match. In the Skill Sequence part, I told you how incredibly high AD ratios have Riven. A full stacked Bloodthirster gives you +100 AD, while Riven has 103,5 AD at level 18. It almost doubles your AD. It's not only the AD ratios why it's so important,but with the +20% from your ultimate increases it to +120 AD, what's three times the AD what a Last Whisper gives, for only 3k gold. With the AD, your farming gets surprisingly easy. There's 20% life steal for addition, makes you an insane lifesteal-absorb tank. After you bought this item, stack it up as fast as you can, to have crazy damage(Mobafire doesn't count Bloodthirster's passive, the +40 AD and +8% Lifesteal, so with this build and the +20% from the ultimate you have 397 AD).
  
• The Brutalizer scales with Riven so much. It gives you AD with a great prize-value ratio, and you receive 10% cooldown reduction and 15 armor penetration.Only what's needed. If you want early advantage and power, this is the best item beside Doran's Blade. I prefer buying 2 Doran's Blade and then the Brutalizer, to maximize my early-midgame strength.
  I usually don't build Youmuu's Ghostblade because what bonus it gives after the Brutalizer isn't so valuable. I mean critical strike chance, attack speed and movement speed isn't worth for another 1350 gold.
  
• Hexdrinker is awesome against AP top. You won't give a kill anymore on toplane. Recommended against Karthus. Maw of Malmortius makes a nice lategame item too, gives you more AD on lower health.
  
• Wriggle's Lantern isn't bad at all, there are lots of Riven players who start with it. I dont usually build it, but sometimes it's nice. A great choice against Gangplank and this kind of characters, where armor is needed, and it helps you to kill some wolves or double golems while your lane is pushing. A little gold and experience advantage is always great.
  
• Phage It's just needed. Health, AD is great to increase your tankiness, and that slow proc doesn't allow the enemy to get rid of you. Later, with the Frozen Mallet, you become a tank, with the 100% slow chance. You're like a Skarner, stick on the enemy and doesn't let him go.
  
• Sheen or Trinity Force is considerable to take, because Riven has a lot of skills to proc the Sheen, and it really hurts with the Runic Blade stacks together. It could be great for Riven, even so I don't usually build it, because mana and ability power is a huge waste.
  
• Guardian Angel You have HP from the Mallet, you need some armor and magic resist too. Nothing is better than a Guardian Angel, and in addition, you can feel comfortably for dieing. After the revive, you'll have your skills again, shield, stun, everything.
  
• Last Whisper Armor penetration for the hyper late game.
  
• Atma's Impaler is a great item to have, I build often, combined with the Frozen Mallet gives you a lot of armor meanwhile you get a high AD item.

This is the most important part, what depends only on the summoner. But I give you a few tips, how I do it, but everyone has own style, you have to create it yourselves.

2 weeks ago, I played 3 games, and I was in top lane against Riven. I played 2 times Irelia and 1 Gangplank. I won,I was at least 3-0 on toplane in every game. Why do I tell you this? Because it wasn't Riven's weakness, it was the summoners' failure. I guess a lot of Riven players have this problem, that's why I explain it. I was premade with our jungler, and asked him to gank level 2, because we knew Riven's a big threat. When I saw him coming, I went close to Riven, autoattacked her, and this was enough bait. She hit me with all 3 and auto-attacks. She tried to retreat, but without jumps, we got our first blood. We pushed the lane fast into the turret, to let the turret shoot down her exp, I bought some Doran's and denied her very hard. She couldn't do anything. This was in 3 different games, 3 different Riven players, but all of them got behind.Riven has a strong early game,Broken Wings with the 2.1 AD ratio is pretty strong, every one of them knew that, and this is what I used against them. Riven's problem is that if she gets behind, she can't get kills, she cant farm as well as she should, it's hard to get back to the game.
To avoid this troublesome situation, I have some tips.

• Before level 3,don't go too offensive. Farm with your auto-attacks,or when the minion survived your attack, it's okay to use 1 to get the gold.You still have 2 jumps to retreat on ganks. Be ready for an earlygame gank. And also, you're stronger after level 3, there's no need on level 1 agression.This is the only counter to Riven.
• Don't push early game. The same reason as above. You're nocost, but this doesn't mean you have to use your abilities. It's just about don't fall behind from early ganks, and you'll be unstoppable. Especially, don't push in the purple side. On that side, you're so easy to be ganked, that it's unbelievable.
• Buy at least 1 ward every time you go home. On the purple side, put it into the tribush, on blue into the river. Don't die just because of that 75 gold. Also, if they see that you play safe, and every time they try to gank you retreat, they'll give up, leave you alone, meaning that their jungler is more likely to simply farm the jungle or gank other lanes.
• Riven is a good jungler. They use her even in competitive plays as jungler. You can take advantage of this, when you pushed your lane into their turret, you can farm behind the turret, you can easily tank them with the shield of Valor and the mastery, and also, take down wolves or double golems. I take a bonus ward often and put it to the enemy Blue/Red buff(depends on the side), and ask our jungler to come, defend, I don't want to get caught 1v2. Meanwhile, I ask our mid champ to arrest their mid champ. It isn't fun to be 2v3 in their jungle. Actually, Blue and Red buff both is good for Riven, helps a lot to control the lane. Only go steal when you have your flash ready. Counter jungling is only useful if you can get away with the buff.
• Always analize your enemies' item builds and that who's got gold leads, see who's stronger at the moment, and initiate fights with this knowledge in your mind. You can initiate easily with a Flash Ki Burst combo.
• Try to burst down carries in fights. You have insane AD with insane AD ratios, you can execute squishies very fast.
• Farming. It isn't hard with Riven, the 2 Doran's Blade help you out. You have high AD, so your attacks can secure the last hits. I have 3 styles, how I farm in midgame.
  The first one is simple, auto-attack minions, and if the attack's timeing is wrong, I quickly use to get it. It's almost instant damage.
  The second thing I do is that I go to the casters, use 1 or 2 , and last hit them with W. Remember, the 3rd use of Broken Wings is way slower than a Ki Burst, and Ki burst also has higher damage. It's easier to get the last hits with the W. Meanwhile, a few Swordsman Minion might be on low health, to quickly get there for the last hit, you can use Valor.
  The third way of farming is that you get one auto-attack to every Swordsman minion, and use the same ability order than in the second way I told, and after that kill the casters with auto-attacks. Remember, swordsman minions are more gold than casters, so if the timing makes you to choose which one to last hit, always choose the swordsman(and the tank if available).
• Dont let the enemy champion to last hit. You are pretty strong, be dominant. If the enemy champion comes, stun him, hit him, to let him feel your love. You can use a few Q hits on him, but be careful, it can be a bait. If you have a few jumps always remaining, you won't get in trouble. If the enemy is about 60% HP and he comes closer, you can get your full combo with R,Q,AA,W,Q,AA,Q,AA,E,AA,R, ignite. This will definitely get you a kill. If he doesn't come, you have free farm.
• Execute enemies with your ultimate. Your ultimate has a relatively low cooldown, if you see someone on low health within range, just pop it for the wind slash. Calculate, that this skill has a travel time, so don't use it for enemies on the edge of your skill. Try to get closer, or leave them alone.
• Kill Wraiths. Ask your jungler to let you kill your Wraiths level 1(only works for purple side), they'll respawn fast. With your Q, you can kill them fast, and you have a half level advantage on toplane. Later the game, always kill the Wraiths when you're around them, because you can clear them fast with the AoE abilities. It means a lot of gold and experience advantage.

In summary, Riven's a very mighty character, I hope you enjoyed this guide. This is my first guide, so let me know if something is missing, any constructive comment will help me. If you have questions, ask'em, you can ask here, or add me ingame.
Thanks for reading!