In general, fighter classes are easier to start with, since they have higher initial hitpoints and can bring down most opponents with the standard hack&slash method. In contrast, my lvl 1 thief has to run rings round them while pelting them with missiles just to bring them down. Ditto for mages (although they have Magic Missiles and other spells to draw on).
In the beginning stages of the game, try to fill up your party with enough fighters/thieves (especially since you're starting with lvl 1 characters), so that normal combat and dungeon exploration won't be a problem. Mages, and their spell-casting abilities, will be essential in the later parts of the game when they gain access to more powerful spells. Since you've already played BG2, I don't think I have to say much more
.