I had to think a lot, because lot of indie games are really really good (Factorio, Rimworld, Enter the Gungeon, Dead Cells, Creeper World, ...).
But I think my top of all is Spelunky. Because through the free version, I discovered an awesome plateformer, who was generating procedural levels, with great gameplay, chaos, and... destructible environment :D It may sounds exagerated now, but for me, when it released, being able to bomb a platform to dig through a platformer level was so great (platformer level who was generated procedurally but still awesome to play).
And through the "full released" version... you keep all of that and you add... COOP !!! With my girlfriend, it's one of our most played game, even today :) We regularly play it, it's "the" coop game for us, that we can't get bored to play :D
I can't wait to see Spelunky 2 here :)