This was a true Sophie's Choice, but I'm going Boogie Nights. It's just a more impressive movie to me, in that it is truly hilarious while being incredibly moving. For sending John C. Reilly on a different career trajectory alone it would be Canon worthy.
TWBB was a masterpiece and important and has two incredible acting performances (and Paul F. Tompkins!) but Boogie Nights has one of the greatest ensemble casts I have ever seen, all on top of their game. That Alfred Molina scene, the scene where they're recording You Got the Touch, the Donut scene... I just have to go with my gut, and my gut says Boogie Nights.