The characters themselves depend upon how strong you have made them. Your best bet is to take the best two of each basic class. If there aren’t two then take another from a different class. I usually have two Sword users (the main character and a flier), two Healers (they all tend to get bumped-off fairly fast without someone to look after them!), two Magic-users (one who is best with Freeze, one who is best with Blaze), two Warriors, two Knights and two Archers. I also usually plan my battles ahead and keep to a few basic strategies, outlined on my tips page.
My Force often ends up as follows: Bowie, Sarah, Chester, Zynk, Karna, Kazin, Slade, Peter, May, Gerhalt, Jaro and Chaz, but it really does depend on who I build up. Any one of the characters is potentially brilliant, but some take a little more time and effort. It can be fun to try out different combinations and see how they work. Try making a Force that uses as few ranged fighters as possible… try without magic users… try building all your characters up to the same kind of stats (and then face the difficult decision of who to use!).