I have a few games I really liked, but haven't thought about them much since. These are Portal, Bastion, Dust: An Elysian Tail, Megaman X and some others I don't remember.
But if I'd have to pick a favorite, I'd have to pick two (or three). Civilization V, Fallout: New Vegas and Undertale. Civilization I probably love so much since it was the first 4X I properly got into, and still play to this day. Fallout: New Vegas was just such a good RPG, and the graphics didn't bug me as much as in FO1/2.
Undertale seems like just the greatest game. I guess I can't judge it as a game, since I've only seen let's plays and streams, but man, that game is something else. I'm kinda disappointed that I didn't play it by myself, but I really wish I had. It was a great experience even though I wasn't the one playing the game.