DMC1 is easily my favorite PS2 game. The gameplay is airtight, the boss design is amazing, the level design and graphical presentation are superb and the game is just overflowing with style and atmosphere.
DMC2 is apparently a hell of a lot less impressive. I haven't bothered to pick it up because everything I've heard about the game has been extremely negative in nature. I might give it a chance at some point, but I'm not counting on it.