I've been wanting to see more of his films, There will be blood and Boogie Nights in particular.
Personally, I think
Boogie Nights is phenomenal. It definately put him on the map.
There Will Be Blood is, in my opinion, a bit of a masterpiece, but be prepared for a very long film that develops slowly. It's also the darkest of his films. The character of Daniel Plainview is one of those few instances in film of a truly vile soul beyond redemption. My heart broke for that poor child that he raised. Daniel Day-Lewis never ceases to blow my mind and this is one of his finest performances. It is rumored that the actor that was originally slated to play the role of Eli Sunday that ended up going to Paul Dano left the film because he was so intimidated by Day-Lewis' intensity on set. When he takes a role, he doesn't f**k around.