For me personally:
Baseball Stars 2 (NES), which lacked the create a player feature.
Toe Jam and Earl 2 (GEN), what on Earth was this supposed to be?
Rush 2049 (N64), expecting Midway to follow up Rush 2 with more tracks modeled amazingly well from real cities, instead getting a load of crud.
Perfect Dark (N64) was a disappointment, because it was no longer a Bond game.