I would barely give the edge to Gamecube because of some of its library, and accessories like the Game Boy Player.
One of the things that makes me sad about the Dreamcast is how many of its great games needed to be imported from Japan, and that there were over 100+ games being developed when the DC was discontinued, some which were games that eventually came out on the other 6 gen platforms. It makes me wonder how well the DC could have done with that library, if other systems' libaries would suffer as a result, and if there would have been a DC2, had Sega not had so many financial problems.