I mentioned Rifts before, but that’s hardly everything in one book. Rifts has literal stacks of expansion books. So that’s out.
Ok. Let’s try again.
Fantasy. Dungeons and Delvers Dice Pool Edition. It’s simple, and yet characters are very customizable. Stats and skills are all a D4, upgradable to D6, D8, D10, and capping as a D12. Roll a stat+skill and beat a target number, often with other dice added for all those customizations. Pick the best two dice rolled as your result. 1-handed weapons all do 1 damage, and 2-handed weapons do 2 damage.
Sci-Fi. Star Adventurer. It’s OSR Star Wars in just 35 pages, costing only $4 for a PDF or $9 for a book. Leveling up is two rolls or one pick on a random table instead of a fixed table. Powers are used just like skill checks, but each with a cooldown period if you fail a roll.