but for one thing i think ranger has to be the number one class for soloing, due to these aspects:
they have the advantage of Light Heal, which is specifically designed to let you heal yourself in combat, so you dont need a healer as much. this could take up a large part of the healing requirement YOU NEED for soloing bosses that arent 50 levels below you (it doesent matter the star/difficulty rating: the scenario i am describing is a level 65/70+ soloing a catacombs boss like the Avatar of Donn, Incarnation, Kiara or Ducarius)
They are one of the most powerfull DPS'. this may just be an opinion, but i think it is so - Rouges are not far behind, though. perfect for doing damage, so why not perfect for a boss fight?

BOLAS. for anyone who knows anything about fighting bosses, thats enough said.
Steady Aim. its a skill that i use on my ranger to improve his...*everything you need to kill stuff*

After all that i must say warrior comes next in line. they are the most balanced class, with lots of vitality and great armor.
third druid, i think they are a nice class to use on bosses your own level (obviously in the current stat of the game this will never be possible with ones like Rockbelly, Dreadroot etc.


lastly rouges and mages are only good for damage, they wouldnt last a second trying to solo a tomatoe boss though

Thanks for asking, and i hope this has helped.