Mobile Suit Gundam Zeonic-Front. (Which was Gheiren's Greed if I'm not mistaken) Because of the slowness, and realism in the game. The mobile suits aren't powerful until you get massive upgrades, and the fact that you have to strategically plan out each mission is wonderful.
Mobile Suit Gundam: Crossfire (Tagret in Sight) For the same reasosns, this game, although disappointing to most PS3 players, was wonderful. The upgrading system was a blast, and it brought a lot of Zeonic Front with it.
SD Gundam X (and G-Next) I cannot get enough of these games, I love the grid style tactic of the game, and it's fun to see so many mobile suits in one game. It's a shame that they cost to much to make English versions.
Mobile Suit Gundam VS Zeta Gundam: This game's story mode was fun, I loved the what-ifs.
Mobile Suit Gundam: Encounters in Space. making your own pilot is so much fun! and seeing so many mobile suits in 3D was awesome!
The game I want to play, however, is Universal Century Climax. The graphics are beautiful cell shades, and it goes all the way to F-91, which I have yet to control in a game.