A three way tie between Altered Beast, Super Mario World, and Mario 64.
Altered Beast may just be in there for nostalgia's sake, being it was the first console launch I participated in. I do however remember having quite a bit of fun with it and beating it over and over and over and over and over and... yeah...
Super Mario World. Man... I think the only game to ever beat this as a launch title is the next one on my list. But shit was this thing amazing. Truly unparalleled level design, gorgeous graphics, amazing sound, tons of little extras, polish to the max. This would probably be my favorite but....
Super Mario 64. To this day I don't think there is a launch title that compares. To plop down your $260 and walk away with this back in September 1996.... Hell, I think this took up all of September AND October. I never once questioned my N64 purchase, despite being my least played system of that gen. Mario 64 was quite honestly worth a $260 purchase all on its own.
That's really it. Zelda could enter/takeover here, but we'll have to see. Even if I were getting a PS3 I don't see anything that would come close. PS1/PS2, nothing. Sega Saturn, hell no. Dreamcast had lots of Soul Calibur, but still not on the same level. So yeah, those are my answers.