Museum of Modern Art
MOMA is huge - five stories - filled with modern classics. Picaso, Pollack, Van Gogh, Wyeth, Dali, Matisse, Warhol, Arbus, Calder, and so much more.
Skip the $20 entrance fee by going after 4:00 on a Friday. The place gets pretty crowded, but you can still get right up close to the art.