Barely worth mentioning due to the ubiquity of the sentiment, but this decision is unbelievably difficult. I've seen arguments on the thread that really sparked my interest.
Regarding the ensemble cast of Boogie Nights - Far superior. No argument. I was kind of bummed that Thomas Jane, Don Cheadle, and Heather Graham didn't get as much discussion as I would've liked. Aside from this being the best Wahlberg performance, John C. Reilly's loyal/lovable idiot, Burt Reynold's cheeky and paternal omnipotence, Julianne Moore gives once of the great performances by an actor in cinema history. Together they have a chemistry that almost feels like a milieu for the audience to be a part of. TWBB has the challenge of primarily being about such a select few characters. Dano and Day Lewis create such stark opponents because they're both so willing to be snakes to get what they want. They're the same type of person, just with different motivations. Despite the success of pulling off this dynamic with perfection, it is less of a testament to PTA's nearly unrivaled ability to form characters.
Devin said that he couldn't imagine a canon without Boogie Nights, and god damnit, he's right. Amy argued about this being the seminal PTA movie in terms of the life it injects into character's, Hollywood, and it's film making in general. I think that this is a movie that makes you appreciate the film making while you're watching without distracting you. Very few directors can show off their genius and keep you completely in the moment. There Will Be Blood on the other hand, isn't as socially important. Devin talked about the mainstream acceptance of porn, and Amy was absolutely right in saying that there are other movies that talk about capitalism in similar way.
I would sooner watch Boogie Nights rather than TWBB, but I think that speaks to the emotional investment that the latter requires. It really is a film that impacts me on such an intense and visceral level. It is a film that is structured and executed in tandem with Plainview's growth and ultimate downfall into a precise dichotomy of inadequacy and resentment. Similar to what Devin was saying, the last moment is the payoff for the journey that the film earns. Side note: this is a grueling movie, but unlike the Revenant, is asserting so many profound notions and has such intriguing and well-crafted character arcs that it is almost impossible to not see it through. Boogie Nights has a similarly earned finale, but feels like some threads aren't payed off as satisfying as TWBB (Roller Girl, for example).
The only thing that is helping me decide is the thing I always think about when voting here or discussing the merit of certain pictures at large. I love movies with such an intense passion that it almost feels like betrayal to pick one, but Spielberg tells a story about the first movie he ever saw (Cecil B. DeMilles' The Greatest Show on Earth), and how it emotionally impacted him. I can safely say that amongst the list of movies like Schindler's List, Raging Bull, City of God, Raiders of the Lost Ark, Blue Valentine, Do the Right Thing, and Dancer in the Dark that have affected me on such a deep emotional and mental level, that TWBB is right at home. That being said, I still haven't voted. Need to sleep on it. With such an expansive canon, it seems almost unreasonable to have to choose. But such is life. First time posting on the voting forum, so I'm sorry if this is too lengthy.