Two for me (and one is quite fitting given the recent post about the 3D remake of it in the news section):
1.) Zelda 2
Don't get me wrong, I did love the first Zelda game and spent ages with it, but even back then there was something disappointing about the graphics and shallow gameplay (not shallow game, just shallow combat, 4 direction movement, etc.). I was an avid fantasy reader from a young age and while Zelda 1 sort of scratched that itch, it fell short in a lot of ways. Then Zelda 2 came out. I wasn't this chubby little one note kid anymore, I was actually totally in control of this character fighting vicious creatures, delving into devious mazes, etc. It was, to use a much overused word, Epic.
2.) Castlevania 2
Looking back, I'm really not sure how I got through it given the beyond horrid translation (I'm guessing it was thanks to Nintendo Power), but I lived this game for months. I even bought the crap book based on it I loved it so much. The mix of RPG and action game, the mind blowing music, the day/night system, the (too-)cryptic puzzles, the gothic and spooky atmosphere. It totally blew my young mind.. even if the end boss was a total wimp (by any games standard, never mind by Castlevania standards).