My current import obsession is Another Century's Episode (A.C.E.) 3: The Final. It's almost as good as Zone of the Enders 2 (one of the best PS2 games, if you didn't know), with the added fanservice of all those mechs from a bunch of Japanese Giant Robot cartoons.

Personally I'm a huge fan of Eureka 7. ACE3 features 8 units from that series alone. Of course there are Gundams as well--can't have a crossover big-mech game from Bandai/Banpresto without Gundams. A few folks from Macross, and several from shows I'd never seen or heard of previously. In all I believe there are over 100 playable mechs. Great fun.