That's not a very inclusive poll at all, is it? If the implication is that anything that isn't on the list isn't enjoyable, I might just have to take exception to that misconception.
In the past 3 years, huh?
Maybe Marvel Versus Capcom 3 in no small part due to it putting in so many of my favorite Capcom game characters (really, I was sold at Amaterasu, Felicia, Dante and Zero but RE5 Wesker truly sealed the deal) against some of my favorite Marvel Characters (again, they didn't have to go far beyond Deadpool, Thor and and Doctor Doom but having She-Hulk in a fighting game in an official capacity almost makes up for Rainbow Mika being left out of the Street Fighter series for so long), all in an accessible yet deep fighting system.
Then again, Soul Calibur 4's character creation mode let me pit Guts from Berserk up against Cattleya from Queen's Blade and Ichigo from Bleach against 'StarKiller', so that goes a long way towards making it one of the most fun fighting games that I've played in the last three years.
Anyway,if I were to answer the question that this thread's title put forward, the fighting game that I've had the most fun with is Darkstalkers: Vampire Savior (with Street Fighter Alpha 3 coming in a close second and Sega Fighter's Mega Mix coming in third). It's a Monster Mash and, as such, it's a graveyard smash. I don't need to say more than that.