Gunn. It's not even a question for me.
People like Riley, they're military trained, trained in a room with a bunch of other people. It takes a while for them to be out on the street and get rid of that 'ridgedness' that comes from non-on-the-job training.
Wesley...I don't think Wesley learned anything while he was at the Academy, given his stint on BTVS where he was begging for painkillers or to be knocked out after he was hit once. He learned on the job, in the streets, but he was still more of a book guy and when they moved to W&H he largely went back to being that guy, choosing to fight with smarts and magics rather than brute strength.
Wood, I assume, learned from Crowley (heh), but while he might have learned the moves of a Slayer, he didn't have the Slayer strength to back it up. I think that might have held him back, slowed him down, instead of trying to show off his own strengths.
Gunn grew up on the streets. He had to learn fast otherwise he would have been dead. He had to adapt, he had to outthink whoever he was fighting and he had to not question himself or wonder if this was the fight he was going to lose. I think it's this quality that the others lack - that ability to think on their feet, to NOT think about form and how their arm is positioned. Gunn goes with the flow of the fight, he adapts to the fighter rather than trying to force his own style on someone who wants to eat them and doesn't give a good goddamn about proper form.
Gunn had to fight to survive LONG before he found out about vampires and the rest. He wins hands down every time for me.