Flash
Is a really good summoner skill because you can use it in so many situations like escaping from jnuglers,closing gaps and surpirse carries with Flash and a follow-up Infinite Duress to take them down quickly.

Ignite
Is the best offensive skill you probably can have and it increases the kill potential on carries and gives you some more lane pressure.

For Masteries I would use 9/21/0 for getting as much bonus defense as possible and get the damage reductions you need if you have to facetank carries in teamfights.
The 9 points in the offense tree are a nice bonus for your skills and your passive because they all can use the spell penetration and you get nice damage boost while Ignite is on cooldown.

Greater Mark of Magic Penetration
Is the best way to get damage on Warwick because 2 of your skills, your passive and items like Wit's End benefit from it.

Greater Seal of Armor
Is a nice source of armor and prevents losing trades to Bruisers in lane , also decreases taken damage by mininons while trading with the enemy top laner.

Greater Glyph of Scaling Magic Resist
Is really good for Warwick becasue he doesn't build that much Health to counter Magic damage so he can still survive AP-Carry Combos late game.

Greater Quintessence of Movement Speed
Has so many advantages but the main reason is to get easier to your enemies while roaming and chase them with your Blood Scent. You have also an advantage in lane for dodging skills or escaping ganks.

Start:

Boots and 3 Health Potion
To get some lane sustain and movementspeed for being more safe in top lane from damage and ganks.


Core:

Mercury's Treads
Are probably the best teamfight boots because of the tenacity to reduce all the CC while tanking the enemy team. The magicresistance is an easy source to prevent getting bursted down by the AP-Carry.

Wit's End
Is such a good item for Warwick becasue he can use the stats so well. His Infinite Duress procs the On-Hit damage five times and gets 210 extra damage this way by just building this cheap item.
The Attackspeed stacks with the proc itself and also allows hitting your Eternal Thirst
more often ----> more sustain and easier trades.
The Magicresistance is a nice defence bonus and can get really strogn if it's stacked 4 times.

Glacial Shroud
Is a good item which solves your mana issues because of the high mana costs of Hungering Strike and even allows you to spam it.
The cooldownreduction is strong on Warwick because you can get such a low Cooldown on Infinte Duress which means easier and efficienter roaming and more Hungering Strike's for lane sustain.
The armor is the main reason to build it and espiaclly if u upgrade it into a Frozen Heart you can't get killed anymore in lane and be really hard to deal with in teamfights.


Teamfight items:

Frozen Heart
Is such a good item for countering the enemy AD-Carry and and let Bruiser desperate on you because 99 armor is so much damage reduction.

Spirit Visage
Is so actually not that much of a team fight item but it makes you a 1 on 1 monster because of all the sustain and just fits Warwick in all its stats.
The last reason is that you get more cooldownreduction do reach the cap in combiantion with your masteries.

Sunfire Aegis
Gives you Health you really need at this point of the game and some armor for being really really tanky while jumping in the enemy team.
The AoE damage is underrated because it's like a AoE Wit's End proc every second and benefits from the spell penetarion of runes and masteries.

Madred's Bloodrazor
Is your finishing items if your team doesn't have a high damage Jungler,AP-Carry and AD-Carry and it allows you to shut down the AD-Carry quickly while tanking the enemy team.
The passive of the items gets procced 5 times with Infinite Duress like Wit's End
and adds just soooo much damage to one skill.
Attackspeed is used for same reason as the Wit's End one.


Situational:

Sorcerer's Shoes
Can be bought if you dominate your lane opponnent and you want to snowball your lane. Scale with 3 of your final items late game and 2 skills + passive.

Doran's Ring
Is sometimes a good buy early game for increasing your Hungering Strike damage and the Mana regeneration to use it more often.
Can be bought if you have hard time keep your mana and health up.

Chalice of Harmony
Is worth a buy against AP's like Kennen/Vladimir/Mordekaiser if you really need to spam your Hungering Strike to heal their damage and counter their sustain skills or Will of the Ancients.

Ninja Tabi
Against heavy Bruisers early game like Renekton/Riven/Irelia/Lee Sin for not being dominated early game and deal with the damage pressure of those guys.

Shurelya's Battlesong
Is a good item at all but it shouldn't be bought if there is already one on your team because Warwick doesn't wants items without resistances.
Helps Engaging and your team can follow up your Flash + Infinite Duress.

Frozen Mallet
Can be bought as last item if your team has already enough damage and you just need to pick out the targets for them.
The Health in combination with the other 5 resistance items makes you nearly unkillable.

Guardian Angel
If you are fed to replace a tanky Item and keep snowballing to win.
Nice mix of resistances.

Infinite Duress
Should be maxed everytime you can because of the damage and the lower cooldown.
Main reason why Warwick is such a scary roamer.

Hungering Strike
Helps you win trades, keeps your Health up and does a lot of damage to tanky Bruisers becaues of the % damage bonus.
Should be maxed as first basic skill.

Blood Scent
Is another skill helping you roaming and even escaping. Benefits Warwick more than Hunters Call and should be maxed second.

Hunters Call
Is like a one point wonder skill early - mid game and a strong late game steroid that even benefits your AD-Carry.
Should be maxed last because it is strong enough for Warwick itself at rank 1 and scales worse than the other skills.

Winning:

In General Warwick is winning against Tanks like Cho'Gath, Shen and Nasus because he can just deny their farm and completly ignore their low harrass.
Additinally he outsustains all of them.

Notice: You can cancel Ultimates like Stand United and other channeling skills by using Infinite Duress.

Losing:

Hard matchups for Warwick are basically matchups with a lot of harass because you can't sustain the damage with your Hungering Strike and lanes that can kite you with slows.

The worst champions to play against are probably Yorick and Kayle because both can harass you without you can counter harass them (slows) and can't sustain like they can.
Also Olaf does pretty well against him because he can break Infinite Duress with his Ragnarok, can win trades with his true damage Reckless Swing and even can sustain nearly as much as Warwick with [Vicious Strikes]] and a Vampiric Scepter.

Another way to counter Warwick is break to break CC and reduce his healing what means Quicksilver Sash or Banshee's Veil are strong buys against him and a well timed Ignite can prevent his sustain in a duel ----> Warwick's strenght is weaken by a lot.