Yeesh, the guy just wants mech games. Xenogears has mechs, and if he'd like it or not, just leave it at that. Well, I live for mech games, so here's some of my favorites:
Macross Triangle Frontier: An amazing game IMO, tons of content and missions, really fun gameplay, switching between three forms and fighting tons of enemies with missiles flying everywhere. Slight mech color customization, and stat upgrading. Great graphics, and there's an academy story mode, but the game is Japanese. There's a good translation guide on GameFAQs, though.
Another Century's Episode Portable: I love the ACE series, and this is like a pocket ACE3. No real story, but it's lengthy and has lots of units from many different mecha series. Good action game, and can be pretty challenging.
Armored Core: If you can get used to the controls, they're excellent PS2 ports. AC3, AC Silent Line, and AC Last Raven. Definitely some of the best mech customization games out there. AC Formula Front was made for PSP, and easier controls, but they tweaked some things and made it 1 vs. 1 only.
Gundam Assault Survive: This is practically like Macross Triangle Frontier, though it came first, and is more ground-combat based. And Gundam, of course. But their gameplay is actually very similar in a lot of ways.
Gundam vs. Gundam NEXT Plus: Great 2 vs 2 3D fighting game with tons of units to choose from, each with unique movesets.
Carnage Heart EXA: A game about programming mechs. You can also control one manually, but it's based on 3 vs. 3 mech AI battles. If you like the idea of programming automatic battle mechs, this game's incredible. It also has a visual novel story, which is pretty good.
Battle Robot Damashii: Another game with very similar gameplay to Macross TF or Gundam AS. More like Gundam AS with ground based combat, but this game has many different series like ACE, and actually, no Gundams.
Danball Senki: Not really mechs... But small mech model kits battling. Highly Japanese, however, but it has a very long story. The combat is good, though, and you can customize your small mechs with tons of parts and weapons. It's an action RPG. Go for the expansion, Danball Senki BOOST. And there's a sequel, DS W.
And honorable mentions, other mech games are Gundam Memories, Gundam AGE Cosmic Drive/Accel Universe, Kisou Ryouhei Gunhound EX, and any Super Robot Wars on PSP if you like strategy games and can deal with all the Japanese.