Millennium, but only because I'm a huge Star Wars nerd and I like to pair it with Falconer's Flight.
After lots of experimentation (Magnum, Nugget, Columbus, lots of Columbus, Centennial, Warrior, etc.) I have decided that I like Chinook for bittering. Some say they don't like the "sharp" bittering it provides, but that's exactly what I like.
I use Summit. To be honest it really doesn't matter for a bittering addition as long as it's relatively high AA so you don't have to use a cartoon of it.