When it comes to combat, few match the Fighter. He fights using a sword and eventually an axe, and can block attacks with his shield. He has to get close in combat to do any damage, but besides that, he's fairly straight-forward and easy for beginners to use. If you're new to the series, try this guy out.
The Wizard is the only class that's truly skilled in magic. In combat, he can blast his competition away with a few blasts, and outside of combat, he can use his magic for just about anything imaginable. The Wizard is also majorly cheap; just use Flame Dart in combat, and you will NEVER lose. Besides that, it's a solid class.
The Thief is the most interesting class. In Mordavia, there is precious little to steal, and no place to fence your booty, but the Thief's natural skill can still be useful. The Thief can jump over low projectiles and throw daggers in combat, as well as sneak and climb his way out of sticky situations every other time.
The Paladin is the hidden class. It's basically a Fighter, but with a unique set of abilities. It also gets the most powerful equipment in the game, and can easily crush any opposition.
To play as a Paladin, you must either beat the game once and import the save, or import a character from QFG2 or QFG3.