Suikoden. I had ZERO expectations for it. The box art looked kind of lame and I just assumed it was a budget Final Fantasy since I'd never even heard of it. I think I bought it for like $5 used from the rental shop. I popped it in and thought, "yeah, this is a knock-off." Talk about being completely wrong. It had so much depth and personality. I absolutely loved it and wish there was more of that kind of game.
Valkyria Chronicles. I bought my PS3 mostly based on anticipation for Uncharted 2 (which I didn't like) and The Last Guardian (heh). I picked up Valkyria Chronicles as that cheapish game you buy with your console to play because you've gotta have something. Ended up being one of my favorite games of the generation.
People mention Persona 5 but Persona 3 was a game I bought based on recommendations but that I didn't think I'd like. It seemed weird and really niche. I didn't care much for it at first, either, because I was trying to wedge it into my "JRPG" preconceptions and it just wouldn't fit. Relationships? School? What the heck is going on? The more I played, though, the more I wanted to play, and really started to love it for being its own thing.