Yeah, I couldn't answer this poll either.
I've always sorta stuck with Mame-X because of the high Neo-Geo compatibility until the recent release of MameoX, which can run almost the same number of Neo games. Final Burn Alpha is wonderful as well, because of the shooters that are available on it, and the co-op beat'em up games (Alien vs. Predator, Shadows of Mystara). I haven't played Daphne yet (The original media laserdiscs are sorta hard to find, so I'd hafta download almost 3 gigs to play 2 games?! No thanks...).
All of the arcade emus have various strengths and weaknesses, that make it really hard for me to choose one over the other.
MameoX has the highest number of playable games, but can't play large games like Street Fighter Alpha 3 and Esp Ra De.
Mame-X has great Neo Geo compatibility being able to play some large games like Real Bout Fatal Fury Special, and is very configurable when setting up joysticks for each player in a game, but its goofy with early games like Star Wars and Defender, and can randomly freeze when starting a game, if you're starting it after previously playing another game.
Final Burn can play Capcom CPS 1-2 games wonderfully, and has compatibility with great shooters like Guwange and Do DonPachi, but emulates the second smallest list of games available for an xbox arcade emu, doesn't allow you access to dip switches on most games, the joystick configuration menu is very basic, and isn't configurable per game.
Again, because I haven't played with Daphne, I can't comment on it...