The best DMs are a treasure; they can make anything fun.

I think "good" systems (from a DM's perspective anyway) are the ones that make that job take a the minimum amount of effort; If I need a monster, I can make a statblock, but if the system includes a monster manual that has it for me, I can spend that prep time on some other encounter. Likewise, having just enough lore, such that I can grab elements and inspiration, but not so much that I have to exhaustively research it, is a huge benefit.

From a player's perspective, I guess my priorities would be a system that isn't actively frustrating to use, and one that I can have a pretty good understanding of the likelihood of whether any particular choice will succeed or fail. Doesn't have to be perfect knowledge, just so long as I have the chance to make a meaningful decision between my options.

If I'm honest, the literal only reason I still prefer Pathfinder over 5e is because a bunch of my go-to favorite classes don't exist and can't reliably be translated to 5e without a lot of kludging or just outright sawing pieces off and sanding the stumps down. 5e does play rather streamlined, and with a good DM, it goes from good to fantastic; but it's still a matter of 'where are my oracles, kineticists, soulknives, gunslingers, and swashbucklers?'

I just don't like playing base, core, or big four.