For me, probably Metroid Dread.
I went in expecting a 7/10 game to pass the time, but it ended up being one of my top Switch games.
I think what surprised me the most was that I expected the difficulty to be frustrating, but while it was a tough game, it had a similar quality to Retro's Donkey Kong Country games, where even when I died it was like "okay, one more try, I bet I can get it this time" and rather than getting pissed off I just kept coming back for more, as the eventual victory felt so damn good.
I also didn't expect its pacing, level design, and progression to be so insanely satisfying. Reaching new areas, gaining new powers, and somehow always ending up right where I needed to be was like a constant dripfeed of pure dopamine.