for sheer sentimentality's sake: Kingdom Hearts. Otherwise, idk I'm torn between Portal and classic LucasArts adventures (Monkey Island & Grim Fandango). Leaning on Grim.
its takes bit getting into idk, the game honestly gave me a headache for a bit. I really would not say it would be on the top of my favorite games list but it is worth playing. I just really love the art direction and stuff. Also Gwendolyn is voiced by the same chick who does Aegis and it drove me up the wall because she didnt change her voice at all for the role, and Gwen is a bit annoying.