Honestly, I am having a hard time deciding myself. Wildcat has some pretty impressive feats (such as taking on and doing very well against the Injustice Society, almost beating Batman one of the best martial arts practitioners in the DC universe, among others). He has flush knockout power that we've seen man times, agility and speed.
Kingpin, on the other hand, has knockout power as well, great strength, speed and egilty enough to surprise even Spiderman and a great level of intelligence.
Kingpin could possibly outsmart Wildcat here, but Grant still is a master combatant, trained in multiple forms of martial arts apparently. Toe to toe I am not fully sure who has the advantage. Both have great durability, but the edge in that area probably goes to Kingpin. For power, I'd have to go with Wildcat. Speed and Agility are also most likely advantages for Wildcat. Smarts is an advantage for Kingpin.
I really can't figure out a winner for this one to be honest, very hard battle to gauge. I thought this one would spark a discussion haha. I could see it going either way.