I suppose I'd have to say Link's Awakening. I hated the Game Boy when it arrived. The idea of playing on that crappy little screen, instead of a TV, was abhorrent to me (an opinion that hasn't changed over the years).
But while I was on vacation, visiting friends, I played a bit of Link's Awakening, and I had to buy a Game Boy to play the whole thing. I literally hurt my eyes playing through it on that piece of shit. Later, the Super Game Boy made it all better.
Honorable mention to Secret of Mana. It still holds the record for most money I ever paid for a new game - just over $100 ($106, I think), at Service Merchandise. It was the only store I could find that had it. I felt it was worth it, so that's high praise.
edit: oh heck, I missed Doom. Ah, whatever. I can't decide, then.