So, woof, this was seriously brutal.
I love Boogie Nights. Like seriously love it. When I first watched it I just watched it on a base level and just loved it because it was lurid and so funny but the older I got, the more the levels of family and creative integrity started to reveal themselves.
That said, I voted There Will Be Blood because I am hard pressed to think of another American movie that was that exact, that towering, and that stocked with incredible creative decisions from the score to the casting to the production design. It's also one of the greatest slow burns I think I've ever seen in a movie. Like the whole time you are just instantly on edge and you don't really know why and like Devin and Amy said it takes you on this elaborate journey that is incredibly rewarding.
On a personal note (which I know kinda taints my vote) but There Will Be Blood marked the moment where I started taking movies seriously. Beforehand they were just fun larks, but this seriously kicked me in the ass to start looking below the surface and really seeking out other important works to chase the same feeling that TWBB gave me.