orangefruitbat
Adventurer
The only successful mecha game that I've been involved in was run through the Marvel FASERIP system. We all had suits of power armour and it was a futurist sci-fi world with a bit of a Guardians of the Galaxy vibe (note - it was a unique game world, and didn't include any Marvel characters). The FASERIP system did a pretty good job of modelling armour abilities and enhancements without making people overly useless/fragile when outside of their armour (looking at you Rifts mega-damage...).