I almost always have Basch out there.

The way I choose licenses and allocate gear, Basch and Ashe have had the best combination of damage and survivability for me. I often use Ashe as my white mage, though I also have the tendency to hold off on curing, etc. far longer than is often prudent, so she spends a lot of time fighting anyway.

Balthier is my usual third choice. For one because I like the character, but also because while he doesn't seem the absolute best in any one area, he tends to do everything pretty well the way I build him, which makes him very adaptable.

For me, Vaan is a good DD choice for "trash" but he also seems to take a lot more damage than the others, which is a liability in tougher fights. I have Fran concentrate on offensive magics, to bring in when the fights require them. I like Penelo, but I rarely use her except for healing-intensive fights, since the way I build her she's good for healing and status effects but contributes very little else offensively.