I was a big fan of the AC series on the PS1. I'm a giant robo freak and love all of the customization. However, I picked up AC2 a while back for $5 and I can't believe how bad the controls seem after such a short amount of time... it's just so hard to move and aim properly. I've heard that the newer games have fixed this so I'll pick one up someday.
The formula is great, though. Varied missions and the arena battles, tons of customization with several different possible archetypes. What's realy nice is that there's no penalty for buying and selling parts; you never lose money by selling parts. If you don't like your AC, you can just sell everything and start fresh.
My favorite recent giant robo game, though, is hands-down Front Mission 4.