Finally have an answer for this weeks later. The Velvet Underground's "Sister Ray" has at least four classic moments:
1) First 15 seconds. One of the most elemental riffs in rock history.
2) 5:27: Moe Tucker suddenly shifts into double time, and the whole song spins out of orbit.
3) 16:05: after nearly 11 minutes of noise, the band suddenly shifts back into verse chorus mode.
4) 17:13-17:25: the song kicks up one last time, only to end abruptly with just some residual amplifier buzz.
Outstanding, all of it.