Hi, my name is noneoftheabove, and I play on OCE.
This is my first guide so feel free to give me your feedback!
I enjoy playing Riven, as there is so much potential for outplays and snowballing in her kit.

Pros:

Great early game and snowball potential
High mobility
Can single-handedly carry games
Very fun to play
has cc and a pretty overloaded kit
No resource bar + 45% cdr = spam those abilities

Cons:

Hard to play
If you lose lane you can become pretty much useless
Very weak to CC
Can be bursted down
Very weak if the enemy builds a lot of armour

Marks:

Armor pen marks are the best choice on bruisers such as riven. I take one crit mark as just one lucky crit can win you that first trade and snowball you a huge lead. The value of armor pen lost is small compared to the value of the 1% crit. This is because this one crit rune increases your crit chance from 0% (never happening) to 1% (it can happen).
Seals:

I take scaling armor seals as they help out vs adc's late game. This is important as late game adc's will do huge amounts of damage, and if you get kited (which you shouldn't!) you will get killed in no time. As your goal is to get to the back line and kill the carry's you will be trying to fight the adc. Early game the armor doesn't help that much even vs an ad lane. You could take flat seals but I prefer scaling due to the late game help vs adc's. When facing an AP lane you might want to take HP seals.
Glyphs:

3 flat MR glyph are necessary to help vs AP carries mid game. The 6 CDR glyphs give you 5% CDR, but are not necessary as my build will give you 45% anyway, but taking these glyphs will give you 10% at lvl 1 which is good to have. You could take 9 flat MR glyphs if you really need the MR.
Quintessences:

Flat AD quints are pretty much the only option, but other options are also viable. E.g: you could take 2 CDR quints if you want the 10% from runes, or you could take a lifesteal quint for the lane sustain. Attack speed quints can also be viable, but if you learn the fast combo they aren't necessary.

Option #1:

Cunning tree:

You don't need Savegary, as you build ad anyway, and with your passive farming should be a breeze, so take wanderer; it's good for roaming, and chasing/ escaping.

Take assassin, as you are in a solo lane, and riven is good at splitpushing so this mastery synergises very well with riven.

Take merciless, as it works well with any damage dealer, and because meditation is useless on r=Riven.

Put 5 points into intelligence for the CDR, this is a core part of this build, as without this you cannot get 45% CDR.

Take Thunderlord's as this patch it is very good and works with pretty much any champion. Also this is very easy to proc with riven, as she uses aa's and spells to trade. You can easily proc this at any stage of the game.

Ferocity tree:

I take sorcery over fury, but either is viable. The attack speed is not that necessary though, especially if you can fast combo, and the increased spell damage is usually more useful.

Double edges sword, is self explanatory: you do more damage at a small cost of taking a little bit more damage (Riven is mele).

I prefer vampirism over natural talent, as the small amount of AD doesn't really help as much as the extra sustain from vampirism.

Both oppressor and bounty hunter are viable choices, but as you are on the island of top lane, you won't be able to stack up your bounty hunter around the map, as much as a mid laner or a jungler. Oppressor works well with riven's stun and knockup so I usually take this.

Option #2:

Ferocity tree:

Take the same masteries as mentioned for the ferocity tree in option #1, but then continue down the ferocity tree instead of the cunning tree.

Take Battering Blows for the armor pen, and because magic pen is useless on Riven.

Then Take Fervor of Battle or Deathfire Touch. Both are viable, although Fervor of Battle recently got nerfed. Even so it is still easy to stack up, and the stacks synergise well with your pasive, as you will be auto attacking a lot. Deathfire touch works well with your abilities as you will be spamming them all game. I personally prefer Fervor of Battle, but both choices work well.

Cunning tree:

Take the same path as in option #1 but stop at dangerous game.

Summary:

I prefer option #1 as it gives 45% CDR and the Thunderlord's mastery is very good at the moment. I use these masteries every game as they just outshine the other options.

Starting:

The longsword start is the most viable the the moment, but you can still start dorans blade and do wolf camp then back and tp to lane. However do not do this vs a Nasus, or someone who you cannot allow to get any lane presence. I prefer to Start longsword and 3 pots then try and get first blood.

Early Core:

The purchase order for these item's does not matter, but buy all 3 before finishing your first item. Buy Hexdrinker as a first or second item when facing a hard AP matchup. Tiamat gives you damage and waveclear, and Caulfields's Warhammer gives you damage and CDR.

Core:

Finish your Tiamat into a Ravenous Hydra, then your Caulfield's Warhammer into a Black Cleaver, then your boots into Lucidity Boots, and finally build a deaths dance. This will now give you a total of 45% CDR, a high amount of AD, armor shred, and lifesteal. The armor shred from Black Cleaver is very easy to stack on riven as you are using your abilities and auto attacks, in very quick succession when playing Riven.

Last Item Choices:

Usually Bloodthirster and Guardian Angel are the items to finish of the build, but when facing a hard AP lane take Maw of Malmortius. Mercurial Scimitar if you need the active and Lord Dominik's if you are facing an extremely tanky team. You shouldn't need Lord Dominik's as you build Black Cleaver, especially if you take armor pen marks as your runes.

Boot choices:

All the boot enchantments except captain are viable but I prefer alacrity for the extra movement speed. Distortion is the best choice if you are having a hard time in lane and want your tp cooldown reduced, or if you want to make plays with your flash. Furor is also a good choice, but you get pretty much the same passive from your Black Cleaver.

Full build against a balanced team:

This is the build I use 90% of the time and gives you a lot of AD, lifesteal, some armor shred, 45% cdr with masteries, and a revive passive (and the death's dance passive). This is the main build for Riven and what I have found to be the most effective build path.

Full build against a heave AP team:

This is effectively the same build as the main build, except you build Maw instead of Bloodthirster. It gives you pretty much the same stats (20 less AD, but more MR) and a passive Magic Damage shield. This is my preffered build when facing a heavy AP team or lane.

Flash:

Flash is a must have. It enebles you to engage, escape, outplay your opponents, and simply offers too much utility to not take. It gives you a free gap closer/escape, and only adds to the high amount of mobility Riven already has. Always take this spell

Teleport:

I take teleport as my second spell 90% of the time, as it gives you a lot of map control, being able to teleport into a teamfight and clean up. Also most other top laners will take this spell, so you should take teleport to be able to match their map pressure and not fall behind in cs and xp.

Ignite:

Ignite is the only other viable option on Riven. Take this if you want to snowball a lane hard, but you have to be able to use it and kill your opponent. If you can't kill your enemy or worse if they kill you, and they have tp you will be at a severe disadvantage. Note: take ignite when facing Tryndamere to counter his ult.

The Fast Combo:

The fast combo revolves around animation cancelling your auto attacks during your 3 q's to attack faster and get your damage out quicker. To fast combo, first you must be standing within auto attack range of your opponent. Then you must q on your target, then right-click the ground during your q animation, then right-click on your target to auto attack, then repeat for all 3 cast's of your q. If this didn't help explain it well enough watch some of boxbox's video's they explain it very well.

All in combo

There are many possibilities for an all in combo on riven, but I will go through a couple here.

#1, standard combo:

First e - r in quick succession, to animation cancel your ult, then auto attack your target. Then hydra - w to animation cancell your w and stun them, then auto them again. Now do your fast combo, then r (wind slash) to finish them off. This will ensure you do the maximum amount of damage over the shortest amount of time.

This combo should look like this:
e - r - aa - hydra - w - aa - q - aa - q - aa - q - aa - r


#1, gap close combo:

This combo is used if you are far away and you need to gap close quickly to get to your opponent quickly. Start by e - r animation cancelling like before. Then q 3 times to gap close, your last q should hit them, then auto once. Hydra - w to animation cancel your stun, then aa again. Your e should be up again so if you need to e then auto one more time. Either way r (windslash) to finish of your combo.

This combo should look like this:
e - r - q - q - q - aa - hydra - w - aa - (e - aa - ) r

Combo's summary:

Riven has many viable combo combinations, you can't really go wrong. Just remember you can animation cancell with e and hydra, e.g: e - w, hydra - w, e - ult, you can also animation cancell with your last q, but don't do this with your w as you will be stacking cc.
The 2 combo's I have shown you above are the main one's you need to know, but there are many different variations of a Riven combo.

Thank you for reading my guide, and I hope it helped!
This is my first guide so feel free to give feedback.

Now go out and rek some scrubs with Riven!