Probably Hiker Wario. Can't give that much of an opinion on Cowboy Wario because I don't have him unlocked and therefore can't get the vibe of what it's like to play as him.
Also, it feels odd playing as Hiker Wario in the scorching heat of Kalimari Desert 2. Feels fantastic to play as Hiker Wario regardless of seasonal clothing.
Well, I had Hiker Wario as my profile pic a while ago, since I thought he looked quite cool, but Cowboy Wario has his charms too, mainly because it's basically one big Mario Party reference. And you know me; I like references a lot.