Ooh boy! This is a fun one.
Vocally, Defying Gravity is fun, but visually it's nothing special. So much better was okay, but it didn't do anything magical for me. The ones that did, however--
Obviously, One Day More and La Vie Boheme are excellent choices, if not a bit obvious. Nonstop from Hamilton can easily join this list, in my opinion. Bringing the entire cast out on stage, different harmonies, one classical and one modern, they're truly magical. For some less obvious-
Urinetown: Act 1 finale ((Yes, that's the title of the song.))
If staged correctly, the choreograph is usually insane. Multiple parts, check. (Breaks into 4 parts at the end.) Crazy contemporary dancing, double check. Narrator's interlude, especially witty and creative? Triple check. Everyone running in slow motion? Why not. Truly a number to remember. We did it in a little church venue and it was spectacular.
In the Heights: Blackout.
Say what you will, but the chaos in this number is somewhat perfect. I think it's a perfect act 1 finale for a musical about down-and-out souls. It's almost incomprehensible as every character comes together under the umbrella of utter chaos-but it's incomprehensible in a good way. The harmonies are insane. And everyone stopping what they're doing for that refrain-"Look at the fireworks///Light up the night sky". Magical.
For an approach that's a bit subtler, "Light in the Dark" from Next to Normal is haunting and beautiful. Not really a song, but the end of act 1 in Fiddler on the Roof-"Clean up, everyone"-is touching to say the least.
And holy cow, has nobody mentioned Book of Mormon? Multiple parts, crazy costumes, and Elder Cunningham rocking his ass off with some hilarious lyrics in the center of it all-this number is great!