Saz pretty much covered the basics well.
I am usually heavy on spellcasters cause I play them well. It depends on your playing style. Going heavy on tanks never hurts, and you can take and give damage.
If you get a mage, a sorcerer is my first choice because they are easy and have a good number of spells available. I tend toward any spells that will summon or deal good damage (but not area of effect spells - friendly fire is a pain)...
the best way to find out is to go through the game once and find out what your strengths are in your playing style. Then you can develop a party to take advantage of that.
