Chrono, Magus, and Robo.

Oh dear god this is the group from hell. Make sure to give Robo all your magic tabs, and on his own he can heal more then Chrono and Marle can together. Before I made a run of that game using that trick I'd use Marle instead just for the healing.

In Cross I don't think I've stuck with the same party more then once, though I remember my first time using the little alien guy and Grobryc.