I love a lot of American Abstract Expressionist art.
I love the sculptures of David Smith:
And his later Cube stuff.
The photography of Aaron Siskind has always been a favourite of mine:
From the painters, I particularly like Franz Kline, Mark Rothko, Barnett Newman, Clyfford Still and Arshile Gorky.
Jackson Pollock's work really should be seen in person as the reproductions simply don't do them any justice at all justice. The work I seem to have found most emotional connection with when seen in a gallery was probably Kline's. It was almost haunting that I found them very powerful but couldn't pin any specific emotion on them, so I seemed to feel a continual flux of thoughts and feelings as my brain tried to tie down exactly how they were making me respond.
I also love European Modernism and Dada, but I'll save that for another post.