Depends what you're in the mood for:

• I personally love Paranoia. It is very much a comedy game but it can be played with a serious take with the right work. Plus it is just a fun read even if you don't play it.
• Rifts is pretty baller conceptually but I don't have first hand experience playing it.
• GURPS has a Car Wars crossover making it objectively one of the best games, can be played with any level of tech, and has buckets of supplements.
• The Burning Wheel is more fantasy but the book is basically a philosophy of play which could be adapted to lots of things, and the same author wrote Burning Empires which is more scifi but I haven't acquired that one so can't give first hand info on it either.
• Can't forget Car Wars. It is scifi though it is a mix of ttrpg and strategy/board. If you want to drive a car off a cliff while dropping mines on a helicopter as you go by and have your robot gunner leaning out the window shooting lasers at a tank it is the way to go.
• Finally I'll pitch HōL because the game is just so unhinged. It is all hand written and scanned, it's edgy and crass, and is a great parody of roleplaying games. It's engaging and fun to read with great art but realistically quite difficult to functionally play because the rules are intentionally a mess. The Buttery Wholesomeness supplement added character creation rules which don't exist in the base game because it is a mess.