Bonus points for any non-D&D or D&D derivative systems.
My first RPG was Palladium Fantasy way back when my grognard uncle invited me to his groups table. I still don’t really grok it, but it was pretty formative of my ideas of what an RPG could be. The lack of balance and extreme existential threats as part of ordinary encounters included.
Needless to say, I haven’t found a group for this system in a looong while, eheh.
Chivalry & Sorcery
I played loads of this back in the 2nd edition days, but by the time 3rd edition had come out, all the people I’d played 2nd with dribbled out of my life (or I out of theirs) and that, combined with the seriously flawed nature of 3rd’s publication, left me nobody to play with. Now C&S is in its 5th edition, and it’s a powerhouse of a game, but I know of nobody anywhere who plays it. I play it solo only, now. I wish I could find a group.
Grey Ranks
You need to have a very special kind of player willing to play a game that’s as depressing and rigidly structured as Gray Ranks. I’ve never found people matching those traits. I can only read the game and imagine.
Space Opera
Yes, it has some of the most ludicrously complicated rules ever put into a game. (1.5 pages of dense-type rules for handing items from one character to another!) Yes it feels like they just took every space-based SF conceit and crammed it into the rules. But there’s a whole lot of remembered good times in those black books that very few games since have ever come close to matching.
Psi World
At a time when games from almost all companies were going crazier and crazier in complexity, with FGU, the publisher of this game, leading the pack with the insanely complicated rules of (in)famous games like Space Opera, Chivalry & Sorcery, Aftermath, and even games like Flashing Blades or Daredevils, out came the small, unassuming boxed set of Psi World with slim rules that were very simple (by the standards of the time: medium complexity by modern standards) and yet contained within them one of the best systems for psionic powers ever put to paper, and had an implied setting with more depth than you’d expect from the low page count in the main rules box.
I had loads of fun in this game playing weak psis, strong psis (verging on abusive), and psi cops. And now nobody’s even heard of it.