Not sure if I'd call him a mecha, but honorary mention goes to the Big P! He died for the NPCs of an MMO after all.
When I was a kid, they had the tagline. "The Biggest Sci Fi Adventure since Star Wars!" on the Swedish VHS boxes of Robotech...I preferred it to Star Wars I must admit.
Okay, every mecha in BGC 2033 is pure awesome gold, but the D.D Battlemover was the most badass-looking of them all. This thing kicked ass, and it had a nuclear device within to spite-f*ck you if you killed it.