6.11 Update:
Its been a long time since I originally made this, feral flare is gone, Devourer came, ww got nerfed, Devourer got capped, Devourer got nerfed again, and Devourer is gone. WW is still a beast with Frozen Mallet and long live Bloodrazor.

4.7 patch:
4.7 Patch brought a nerf to feral flare, the hp recover doesnt stack anymore, but you get stacks from assists and kills, so what does it mean to ww?
Well, it means more damage.
You get feral flare earlier now, because successful ganks will now give you a stack, and feral flare didn't make ww viable because of the heal, like on Master Yi, at least not the way i build, it made him viable through increased jungle clear time, and tons of damage, and ww still got it.
The only difference for me, is that im tending to buy banshee more often than spirit visage, as spirit visage doesnt escalate as well as before on health regeneration terms, so the CC protection helps me executing my target faster, as i cant tank them as long as before, and im also ganking a little more often, principally mid lane, followed by a wraith steal.

WW is still god tier ^^
http://www.nerfplz.com/2014/05/2014-champion-tier-list-solo-queue.html

Im a platinum 3 player at the Brazilian server (reached Diamond 2 last season, with the old Swain), and my main champion right now is the wolve but i don't play him as most other guides, so i decided to expose a different way to play him.

Pros:

1. Very strong mid game
2. potential to carry a game
3. When you start ganking, every gank should be a kill
4. Can solo dragons
5. Can start with ward
6. If invaded, hard to die due to jungling with full hp all the time
7. Can duel or 1v2 easily during mid game

Cons

1. Without blue, first clear might be really slow
2. Really bad ganking before lvl 6
3. Team might get too dependent on you

I like the mov speed runes quints because it helps your jungle speed and makes it easier getting the first hit to slow people, but overall is a personal preference. Atack speed quints are quite viable aswell.

For marks, atack speed runes, maybe not the best for the first couple clears, but overall i wouldnt use any other rune.

This is one of the secrets to ww being so strong right now: Fervor of Battle
Honestly, I played him a little with 12/0/18, but he felt really weak. After changing to 18/0/12, his ult just makes so much damage. You basically get to do around 200 extra damage, depending on your lvl because of it. If you finished Razor, thats another 20% of his life, and than theres your passive doing some more damage. I once did 100-0 a khazix only with the ult (I had heralds too)

If you are on blue side, get machete and pink. Leave the pink ward at the bush of your red buff, where people usually protect the buff, and go to the enemy red buff. If no one is protecting is there, steal it, (you can solo it, smiting when its 390 hp. Also, you need to start with W), leave a ward so you can see his wraiths and steal the golems too, while always looking if the enemy jungler isnt going to your red. If enemy is protecting the red buff, just get back to your blue buff/frog. After doing golems, you can choose to recall or to get that little thing on the middle of the river.
Than just go frog, do blue, wolfs, wraiths, red, and golems and repeat until lvl 6.

If you are on red side, just start at frog, do blue, wolfs, wraiths, smite on red, and if the enemy top laner is low hp, smite on golems and gank. Otherwise just recall and start on frog again until you get to lvl 6.

Until lvl 6, you will only want to gank if mid or top lane are really pushed and mid health.
After lvl 6, whenever you have ult, try ganking mid or top. Bot lane is harder to guarantee kill, so only go if you are sure you can get a kill there, also if you don't have ult, keep jungling.
When you get a successful gank at mid lane, try forcing a dragon, and always keep the dragon warded.
After you completed bloodrazor, gank top(you can easily get a kill) and ask your top laner to assist you on Herald. Get the Herald for yourself, it does tons of damage, makes your jungle a lot faster and gives you tons of trading potential. You might be able to 100-0 some squishies with just the ult if you get the Herald, and 1x1 will always go on your favor (and you might get away with some 2x1 if you smite correctly and they dont have much cc)
After you have boots lvl 2 and Frozen Mallet, if you touch an enemy who doesnt have movement skills, hes likely dead. After 4 or 5 hits he will probably be under 50%, and then, even if he does have movement skills hes dead.

This is a part where my guide gets different from most others, and that is on the frozen mallet. I mean, why would you build a frozen mallet?
Frozen mallet is a item that goes extremely well with ww, it gives you damage, it gives you more health, so you get tankier, and it gives you cc. I see most warwicks building a randuins, wits end or blade of the ruined king, and while they might not be bad choices, i would pick frozen mallet in 99% of the times over them.
Wits End: i used to build it too, but while it does makes your ult take a ****load of life of squishes, it also leaves you squishy, and if your ult gets stopped, you are in big trouble. Frozen mallet on the other hand, gives you a permanent slow to catch enemies, even if your ult gets stopped, and a nice amount of health to help staying alive.
The same goes for BORK, sure, it gives you life steal and increase your ult damage quite a lot, but you have enough damage with ult + razer + Fervor of Battle so i prefer focusing on sticking to an enemy and getting tankier.

Frozen mallet becomes unique to solving ww problem, because he has no gap closer other than ult, so you cant chase an enemy with 51% hp.

About situational items: Pick spirit or banshee for magic resistance item, and banshee should only be picked when your ult is getting stopped too early or they have a LOT of burst magic damage focusing you.

I personally like Deaths Dance, as it gives you some resistance, and works well with bloodrazor, plus it has 10% cdr, that along with Spirit and Frozen Heart gives ww 40% cdr
leaving your ult at 42 sec cooldown. So you kill a carry, end the fight, go push some towers and when hes alive again, ult him again.

You generally want 1 magic resistance item, 1 physical defense item, and 1 damage item to finish the build. You can pick whatever you like of that combination, and might change 1 of the 3 for another defense item.

You have one mission on team fights: as you didn't really gank bot lane, enemy adc might have gotten strong, and you must destroy him, but that ok, you are the perfect man to stop him.

You will only start the team fight, if the enemy mid laner is REALLY BAD POSITIONED, or you can get to the ult range of the adc, and your team can follow up. Once the TF has started, if you didn't start it, go ult the adc, and ignore everything else. Your ult, plus a Q in the end, is usually enough to get the enemy adc to 50% hp, so chasing will be really easier. The support will try to stop you, and the adc will try to kite you, but with permanent slow from the frozen mallet, and the blood scent, plus all the tankiness you achieved, they might even kill you, but that will give your team enough time to win a 4x3, and in case you didnt die, just keep attacking whatever is near to you.

You are great picking people off, but only get a non-priority target on your ult if you are sure there's no backup to get you while you are ulting.