Not Updated For Current Season
Constructive criticism is welcome, but READ and better yet TRY before downvoting. This build is a little different from a lot of the ones I see around but I find that it suits my play style more.
When I'm playing Ren I tend to find that a lot of my damage comes from sitting in the middle of the enemy team and letting my ult burn down their health while my stun keeps them in range and my heal keeps me from dying. This build lets your burn down health while still being incredibly tanky yourself.
Build 1: the AD killer.
My personal favorite, best vs AD. With your Atma's you'll have deceptively high damage and at 4000 HP with 260 armor it takes nearly 10 seconds for a whole opposing team to kill this build. The main strategy for this one is to wade into the middle of the opposing team and then focus down the squishies.
Build 2: the mage killer.
Still tanky, but with more MR and a little more damage. With your wit's end at full stacks you have over 200 MR and armor, and you can deal a lot more damage. This build requires a little more caution than the other build but it makes up for it by being a caster killer.
Build 3: just generally killing.
If your team is already beefy and you need a little more damage output then go for this build. You can't just wade into the other team as much with this one, but one on one you should be able to kill most enemy champs with little difficulty.
Greater Mark of Desolation
Some nice, generally useful runes that work well on anything from a tank to DPS. I don't like to recommend champion specific runes because they're too bloody expensive to get a new set for each champion you play. If I were to make this more specific to Ren I would probably exchange the magic resist glyphs for flat cooldown reduction, and possibly the same for the quints. That said, pick the runes that fit your playstyle, these work well for me.
Not much to say here. We're building beefy and the masteries help with that. The reason I go for rather than is that the 15% spell pen makes a noticeable difference in the damage from your ultimate whereas the armor penetration helps out a TINY bit with your Q and W.
You're not exactly speedy, so having a slow to get them in range of your stun and catch runners is definitely useful.
Deal that last bit of damage or prevent healers from doing their thing while you destroy them. I cannot count the number of kills this spell has secured for me on Ren.
This compensates for the lack of speed, and gives you another escape mechanism in addition to your E. It also helps you keep people in range of your stun and AOE damage.
A good spell on any champion, I like to use it for initiation more than escape. Pop the ult and then flash onto the squishies.
Good for getting to and from pushes, get back to your lane faster. If you find yourself always coming in at the end of a teamfight then this is the spell for you.
Not so much a passive as a way of managing Ren's alternative resource system. You get 5 rage per autoattack, and 7.5 if you're below 50% health. Your abilities get stronger if you have more than 50 fury.
This is your main farming ability. You'll want to stand in the middle of the creep wave for this since it gives you 5 fury per enemy hit (up to 25) as well as healing you for a percentage of the TOTAL damage done (up to a hard cap).
The furious use of this is primarily healing. If you just came out of a teamfight use fury charged cull the meek in the middle of a creep wave a few times and watch those green bars come flowing back. I'll often use a fury charged ctm early game to get a bunch of last hits in at once.
Your main damage dealer. I like to save my fury for this ability since it ups your damage immensely, the Stun is also very nice for ganks.
If you're about to attack a champion you need more than 50 fury. You'll be spending your fury on this ability. Hit it once and watch them fall.
A short range dash that can go through thin walls, but don't forget about the damage which is quite decent. The uses for this go on and on. It doesn't use fury till dice, so don't be afraid to initiate harass with this ability.
If your Q and W are both on cooldown and you need that extra bit of damage to finish off your opponent, use a fury charged dice. Otherwise save it for the other spells.
Your ultimate, and oh what an ultimate it is. You're going to want to use it a couple seconds BEFORE ganking or initiating a teamfight in order to let rage build up. In a 1v1 I like to wait and see if I'm winning before I use it. If the fight is close then I'll hold off on using it till my opponent and I are both low and then use it. Surprise! The almost dead croc has just gotten 9 feet tall and is dealing way more damage. See you after respawn.
The first two items are pretty much the same in each build. You'll want to start off with boots and
health potions (your Q heals, but not much early game).
Your choice of boots depends on the enemy team you're facing and your own team composition, so I usually stick with level 1 boots until I know what I'm going to need.
These boots are generally good no matter what team you're facing. Since you don't have to worry about mana, your spamming of abilities is limited only by your rage levels and cooldowns, these help with the second part.
Pretty much only if the majority of the damage is coming from physical DPSers, otherwise the CC reduction tips the balance in favor of...
...Mercury's treads. These are the best defensive boot option against a balanced team.
Once you're in lane you should start building your sunfire. Since a lot of the damage from this build comes from the aura from your ult, why not enhance that a little bit with the AOE damage aura from the sunfire? The health and armor help a lot as well.
Every one of these builds has a frozen mallet as well, though at differing purchase priorities. The slow procs with your W, the HP let you sit in the middle of the opposing team more easily, and you even get a little AD for your trouble. With frozen mallet, once you get someone in the range of your AOE damage, you shouldn't have any trouble keeping them there.
Build 1 items
I find that I tend to end up dueling a lot when the enemy team is AD heavy, thornmail ensures that you'll win every time by returning their damage back to them.
You've got a frickton of health, lets use that to get you some AD. This gives you 60 AD at this point, and once you get your warmogs that goes up to 80, putting you over 200 AD.
Yes, you are getting warmog's last. Yes I know that this means you might not get up to full stacks, but what you're failing to realize is that the extra health it gives even without stacks is more than worth it, plus you're upping the damage from your atma's. If you really care that much about stacks then you can buy this before your thornmail, but I find an earlier thornmail more valuable, and if the game is dragging on long enough for me to get a 6th item it usually goes on long enough for me to get a fair number of stacks up as well.
Build 2 items
Once you have full stacks this puts you over 200 MR, and the stacks proc with your w. The attack speed is pretty useless on Ren, but it helps with last hitting minions and it doesn't hurt anything.
If you're facing an AP heavy team then most of your opponents will be doing burst damage. That means you'll want to survive their initial blast and then take them down before they can do it again. IMHO this is the best damage item for ren, even though you won't be using the attack speed all that much, the armor reduction procs on your W, so you'll be doing more damage with each hit.
More for the stats than anything. If you want to balance out a little armor into your build then get the GA, otherwise go for the FoN and watch the mages cry.
Build 3 items
CDR, damage, and armor pen. A nice early game damage dealer. You can upgrade this to a youmouu's later, but you get more benefit from finishing the rest of the build as the active on youmouu's is pretty useless for you. I often sell it late game in favor of some other damage items.
Like I said, the best damage item for Ren, hands down.
Damage to kill your enemies, and lifesteal to keep you alive. You can go for an Atma's here if you want to be a little tankier.
If you want to up your damage output you'll want AD and on hit effects to proc with your W. You don't need attack speed since most of your damage will be coming from your abilities.
I would go for a bloodthirster over an infinity edge since you shouldn't be dying much with these builds so the stacks on Bloodthirster should be easy to maintain.
If they're stacking health then go for a bloodrazor, it's a pretty obvious choice.
Defensively you can go for a Frozen heart. The mana is wasted on you, but the CDR and aura more than make up for it. Alternately you can get a for a little more money, but you still get CDR and a bunch of other useful abilities.
You have a pretty dominant early game, so don't be afraid to be aggressive, but not to the point of stupidity. I tend to tower dive a little too much early game, and it has cost me both lives and kills. Don't harass to the point of forgetting to get minion kills either. CS>1 for 1 kills. Usually I focus on last hitting until level 3, and then continue last hitting until a good opportunity to initiate/harass arises.
Gank if someone needs ganking, be sure to be in teamfights as they arise, but otherwise you should be in the fields farming minions all the live long day. You particularly need your sunfire, as it increases both your farming and teamfight capabilities.
Pretty much just stick with your team to make sure you win teamfights. You're not fast enough nor do you have the right abilities to be a split pusher like Sivir or Nasus. Killing champions (and on occasion tanking the tower) is where your strengths lie.
EvilDice has a nice list of Croc combos that I'm going to shamelessly copy here, they're well written and don't change much from build to build.
Slice in to all minions, use Cull The Meek for some quick fury, stun with Ruthless Predator, Dice out.
Build up 50 fury,Slice in to minions, use Cull The Meek, Dice out.
Escaping: Run into bush, wait for enemy to move to the front of the bush (to cut off your escape). Stun if necessary, Slice through the enemy, Dice away.
Chasing/Ganking: Slice through minions if you can't reach the enemy, apply stun with Ruthless Predator, Dice through enemy along escape path, exhaust if you have it, and spam Q and W.
I have a couple more combos that he fails to mention:
Initiating/dueling: hit the stun and then slice to your enemy to hit them with it, use Q and then dice through them to get the extra damage in. Auto attack and use abilities as they come off CD, though it's best to save slice for chasing.
If you're initiating a teamfight you should have popped your ultimate before going in, if you're dueling wait until both you and your opponent are low (if you stay high don't waste it).
Once in the middle of a teamfight you should exhaust a squishy (preferably the AD carry) and stand next to them doing as much damage as possible.
Good luck, have fun
This is the way I play Ren; A little tankier than most, but it gets me good results. That said, builds are just suggestions, I would love to hear how you adapt this to your play style.