Oblivion Orb
Orb is great against Warwick, and I like to rush it. Bramble is a complete bait, as Warwick actually does a lot of magic damage too, and he doesn't have to auto you. He can instead just heal off minions.
Heartsteel
Heartsteel is great in general. You realistically probably have a lot of options, but Heartsteel can pop WW easier, gives more range, and is just generally good.
Swiftness Boots
As with most situations I highly recommend Swifties over Tabi's or Merc's. I think it's much better for keeping up with him in chasing and escaping.