I'll go ahead and start with a game that I find super fascinating, and which never gets talked about:
Mega Man Battle Chip Challenge
This game got a really poor reception when it came out, which was understandable as it was a big departure from the rest of the Mega Man Battle Network series, and also just very weird. But I think this game really stands on its own, especially now that a lot of time has passed.
What's unique about it is that the gameplay is really, really distilled. It sort of takes the formula from Battle Network and isolates it down to just the battles, but then it goes even further to where the battles sort of play themselves out. It's closer to an incremental game in a way. But it's tough to describe, I've never actually played anything else like it. It makes a lot of weird design decisions that I don't see elsewhere, for better or worse.
I think it would have gotten a much better reception had it released 10 years later. The sort of "distilled gameplay" is something that sees a lot of success nowadays (Downwell, Vampire Survivors, Loop Hero just to name a few examples) but at the time it felt out of place. For me personally, I loved it when it came out, and I still return to it occasionally.