I really don't have a traditional first ride... though it seems like a few trips have started with Space Mountain. When I brought my friend, Allie, for her first trip to Disney World I wanted her first ride to be Magic Carpets because I had gotten weirdly obsessed with it on my college program. Well, she hates spinning rides so we ended up on Pirates first, then Magic Carpets. Nice little change of plans that got us some dream FastPasses as we exited Magic Carpets!
My first ride on my upcoming trip will be Mine Train, but first ride with everyone a few days later is Space Mountain.