I have to say Super Smash, but aside from the greatest fighting game to come out for the cube...
Mario Party is a good choice
Soul Caliber (There's nothing like spanking Nightmare with Link's sword!)
Madden isn't bad (but I would say just go ahead and get it for the Wii it's the best Madden ever)
If your willing to shell out some shipping and handling then I would also suggest importing Naruto: Gekitou Ninja Taisen 4 from Japan. It's the same series as Clash of Ninja only it's the 4th one. America only has up to the 2nd one. My friends and I play it almost as much as Super Smash. It's in Japanese of course but you can get translation guides online.