Depends on the level that you are at. It's always going to vary, the main shift being at lvl 41, rdm becoming healer. The others are pretty interchangeable. My favorite DD is Samurai, and you can't beat having a SAM and a MNK in your party. Personally, as far as healers go, i could care less RDM(after 41) or Whm (before OR after 41) As long as they know thier job. Nuker, I would say smn, because then you have a nuker AND a buffer, but if you have only DECENT dd's, then you'll wanna go BLM. And ninja's always make the best tanks, but ONLY the rich ones. So i guess, here's my IDEAL party.
Healer- WHM/BLM
Nuker- SMN/WHM
DD-SAM/MNK
DD-MNK/WAR
TANK-NIN/WAR
MISC- RDM/WHM