If I were to just pick one, I'd have to say Spelunky HD.
If I were to pick more than one, I'd say Spelunky HD, Metroid Prime, Wind Waker, FEZ, Hearthstone, Nuclear Throne, and Monster Hunter 3 in no particular order. There are so many others, and so many more I haven't even played, it's hard to narrow it down.
Spelunky and FEZ are two of my favorites as well.
I've always wanted to get into Monster Hunter (I even got a PS2 modem for the first one, that and the weird online co-op Resident Evil game) - the Souls series is one of my favorites and I recognize kinship at least in the combat system between the two series', but I've never really been able to stick to MH.
Yeah I'm certain I'll love souls whenever I get the chance to actually play one of them. Monster Hunter seems to have similar combat and difficulty, which I'm totally into. Monster Hunter is a lot easier to play if you have a friend or two to do online play, although I've gotten hundreds of hours in solo play.
