My opinion is that mages are unmatched for anything below SM. Mages dominate low level instances for speed, then paladin start to take over. Hunters are ok, they can handle anything but they don't do it very fast. Slowly but surely goes the hunter. Warlocks are almost on par with hunters for being able to solo most all instances thoroughly, but they also have seed of corruption for lower level instances.

I have 6 classes at 70 (no shaman, druid, warrior)that I soloed there before boxing. I've tried boosting with them all and those are my opinions.