The music, the bosses, excellent control, and straightforward gameplay--it was enough to keep me hooked until I'd finished it, which--looking back-- is a pretty high accomplishment for an eight year old. With all of the repetition required getting to that point, it's one of the few NES games that I can pick up and still be any good at.
Super Mario Bros. 2 (NES)
At this point, it's no secret that SMB2 is just a reskinned version of the Super Famicom game Doki Doki Panic, but in spite of that, some elements of the game, like carrying and throwing enemies are now mainstays of the series. Heck, the amazing ragtime music of the character select screen and overworld are reason enough to own a copy this game. Plus, it's a great challenge. I finished it a couple of times back in my youth, but I doubt I'd have much luck if I tried now.