Hm. It's really a toss up between a few games from NES, and a few from the Master System. My memory tells me I grew up first with MS, but I also recall having NES around the same time. Which isn't to say they were released at similar dates. I wouldn't know if they were, I just remember having them together. The main games on MS that I adored above all other MS games were Zillion 1 and 2, some racing game (I forgot the name. Shameful, I know.), and Wonder Boy 3. I played those games like an addict.
On the NES, there's just so much to choose from, but my favorites were Mario 1, 2, and 3. The MegaMan games. The Castlevania games, Gremlins 2: The New Batch (the music was catchy), and a few others.
There was one game where you played as some kid, or wierd green character, that made rainbows that you can use as platforms to collect stuff with, or hit the enemy with. Again, don't recall the name, but that too was an old favorite.
Agreed. The NES MegaMan games were the shit. I played them all several times. Though, the ones that really stuck out to me as ever lasting were 2 and 3.