PS2 has the largest and varied selection and even has a DVD.
Game Cube is small and compact but game selection is Limited at the current time.
Xbox is one of the largest consoles currently out (physically) and it hasawesome graphics yet, same as the Game Cube its game selection is limited.
Dream Cast was good during its release but since they have discontinued creation of the system its been losing popularitly along with all the other sega products.

I also agree with everyone elses opinions about these systems but personally I believe Ps2 is currently the better system at this time.