The D20 based Cypher System game Numenera is what I run and have the most fun in. Takes place about 1 billion years in the future on the Ninth World, what future humanity calls the new Pangea continent that once hosted 8 previous super civilizations that rose and fell.

Character creation is fun, based on the formula 'I am a [descriptor] [type] that [focus]' with what descriptor you give your character giving appropriate bonuses, like Charming giving skills in interaction and starting off with an important contact you charmed before the game. Type is class, like a Nano that can control some of the latent nanites in the air. Focus is something very unique, something that can start an adventure itself, like Talks to Machines or Murders.

Health isn't nebulous HP like it is in dnd, instead split into three pools 'speed' 'might' and 'intellect' with some abilities having pool costs, or some strenuous actions depleting it. I like the feeling of weakening it can give after a day of adventure.

Even if you're not into it I'd get the bestiaries at least, best I've seen so far and easy enough to slot into dnd.