One possibility is the Palladium Books line of RPGs. Specifically, Rifts, Robotech, and Mechanoids.
The "Mechanoids Trillogy" and the old Mechanoids book have a strong storyline, but it's just a "kicker" storyline; it has no details past the starting situation. Lots of good stuff, SDC only (no MegaDamage), no personal SDC.
Rifts and Robotech are both strong settings as well, but the mecha can easily be reused in other ways. One could easily cast the Zentraedi and Invid mecha as Robotic instead of alien-manned.
And then there is the Rifts: Mechanoids book... which brings the original mechanoids back as MDC nasties.
The systems are simple, loved by a large fanbase, filled with excellent illos... but they are also controversial, because the utterly lack any interpersonal skills or abilities for the characters, have an "almost never miss" combat system, and have issues with new character classes having been added in supplements that are vastly superior to older ones.