Anything BattleTech. It's what the 80s thought the future would be.
That's why I love crossover fictions that has someone or a faction use modern technologies or what we think future technology is going to look like with our current modern tech (basically doing what the folks in the 70's and 80's were doing then with us now), which baffles the fuck out of the Inner Sphere and Clans in ripple effects: When done well, it creates some interesting situations in-universe.
I can imagine Inner Sphere/Clan scientists looking at a modern processor and thinking "what the fuck? Did aliens make this?"
"So, this previously unknown mercenary group just appeared from pretty much nowhere. They claim to be from some far-flung Periphery world, but they're evasive about exactly where. Their kit looks like standardized Inner Sphere you'd find on any second hand market, but their observed effects and stats don't match up -- they're too powerful and efficient, which implies imitations. Their mechs are lighter, more durable, and move a hell of a lot faster than even our best designs, and there's no doubt we're centuries ahead of the Houses."
"You're thinking a third group is doing the same thing we're doing? Scouting the Inner Sphere?"
"The evidence leans that way, yes."