I will admit, Butterfree is probably my favourite starter-bugs (i.e. Butterfree > Beedrill, Beautifly, Dustox), but that doesn't say much. I actually quite like Bellossom as a grass-type as well - the way its wearing a little hula skirt probably makes it the most summery of the three.
But Lapras is hands down my favourite out of them three. Great typing, versatile movepool, great design, known for being friendly, actually saved my life in Silph Co. on my first play of LeafGreen too (I was down to 1hp on my starter, all others fainted) by taking me to the bed on floor 9. He also appears in cool locations - Silph Co, that cave in Johto, and appearing in the summer resort in Ranger, so maybe it is summery :)
Basically what I'm trying to say is Lapras >>>> Bellossom > Butterfree xD
EDIT: Dude, no poll?!