Well, I've been playing tons of niche games - try to beat a text-based Star Trek (1971 game), so I'll mention just three which were able to stand the test of time.
- Jelly no Puzzle (free PC-Android game), one of the best puzzle game ever created,
- Outer Wilds alpha (free PC game). I loved it in 2013, it was my personal GOTY. Not so niche anymore since 2019.
- Shariki (free DOS game). It's the first match-3 game, known for its clones - Bejeweled, Candy Crush Saga etc. Again, not so niche nowadays.