Haven't played many mech games. My favorite is probably Powered Gear. Cool mech beat em up in which you can switch out your arm, bottom frame, and mounted weapon with whatever the enemy drops.

Battletech: Armored Combat for the Genesis is great as well. You pilot a Madcat and do campaigns Desert Strike styled. It does get hard from campaign two and onward since you're on a fire planet depleting your life and the first mission is literally a tightly timed one on a map you're not familiar with.

Other mech games I'm interested in are the early Mech Warrior games, Gundam Sode Story 0079, Metalstorm, Metal Warriors, Target Earth, Cybernator, Future Cop LAPD, Assault Suits Valken, Front Mission: Gun Hazard, and Ranger X. Played Virtual-On in the arcades, but sparsely. Was a really fun game from what little I've played.