I loved playing disc 2 because of all the fun and action-packed moments--indeed there is much to do on that disc!! But I voted for disc 1.
I loved starting out the game--meeting and getting to know each character as they joined the team and getting incredibly attached to them, and the FMVs were pretty freakin' awesome on the first disc (what with the motorcycle chase, even the planetarium at Cosmo Canyon was pretty cool! and Aeris' death scene towards the end).
Discovering the Gold Saucer for the first time was great, too (even though you couldn't breed chocobos at that point) and the side quests could be done on that 1st disc too - such as going to Wutai and finding Vincent.
Each disc is pretty amazing, but I guess I voted on sentimental reasons more than anything else. XD I always prefer the beginning of the story to the end--because I know that there is so much more thats left to enjoy. I hate coming to the end of a movie, book, or series, because I know that a good story is almost over.