Mass Effect: Went in expecting a good sci-fi game. Came away with the best sci-fi game (at least until 2 and 3 came around).
Tales of Xillia 2: Sequels to games in series like Tales, Final Fantasy, etc. don't tend to be great. But I liked the characters in Xillia and 2 was on sale, so I checked it out. Xillia 2 blew its predecessor out of the water. Better writing, better characterizations, a focus on actually getting to know your party members, and Rollo the cat. It was all fantastic, and I really hope other Tales games start taking cues from it.
Fallout 4: I was a fan of 3 and New Vegas, so I figured I'd enjoy this too. I did, but far more than I expected. Like Xillia 2, it allowed players to engage with their companions much more than previous games, and that's like catnip to me. Add in a more polished experience than the previous games and the addictive settlement building system, and baby you got a stew going. Plus I play on PC, so thanks to mods, there's always something new to try out every time I do a new playthrough.