well the best casting class is warlock and the best melee class is headhunter. you arent able to use those right from the start. you will mostly use the highest class which is possible with your state of climbing the ladder.
for feeder (first 5 classes) you should start with barb or fighter or eventually rogue. mage can be fine with the right casting items too and outdmg the others. barb and fighter will be the best to make it through with the least healing with tanking armors.
guardian is the weakest and also very limited to gear choice..