Jump Ultimate Stars. It's such an incredibly large game. I've pumped in well over 99 hours into the game (The game clock stops at 99 so I don't actually know how much of my life I've clocked into it) most of which was spent unlocking all the different characters and preset decks. (Not an easy feat, I might add.)
It's a real easy game to pick up and play, something I can't really say for other fighters like BlazBlue, Mortal Kombat, Tekken or SoulCalibur. If I had to compare it to anything, I'd probably call it as close as we'll ever get to a Super Smash Bros. DS, except I enjoy it a lot more than Super Smash Bros.