#1 is hands down 'A Little Priest'
It is not only the finest ACT I Finale for any musical, it is, to me, also the most well-constructed, most clever and greatest showtune of all time.
After that, I would say...
2) Sunday, SITPWG
3) A Weekend in the Country, A LITTLE NIGHT MUSIC
But really, this is an impossible task. All of his Act I Finale's are perfect for each show they are part of, and, since every one of his shows are so incredibly different, every Finale is incredibly different. It's really comparing apples to M&Ms.
"One no longer loves one's insight enough once one communicates it."
The opposite of creation isn't war, it's stagnation.