Best solo class....
It depends on what you want to solo, or how you want to solo.
If you want to be able to solo as difficult a scenario as possible, then Shadow Knight. They can FD to split if needed, they can self heal and take a beating while also doing decent DPS, ending the mob before their mana pool goes out (reason why Paladin and Clerics ect., arent able to solo what SK's can).
But if you want to kill mobs that don't summon, play a Ranger or Wizard. Snare them and bow/nuke down your target.
If you like to be face to face, and also do more dps than a SK, then do a Bst or perhaps a necro (at higher tiers Necro can tank so so, with their lifetaps and pet auxing/dpsing).
If you like a challenge, then do a Wizard, they can blow up mobs really fast, but they cant self heal....
Druids actually do decent dps with their DS and pet, but probably not more than a SK; but then they can port and track...
Warriors are actually not bad at soloing either, you probably need a bit of gear though.
Mages are nice as long as their pet can take the heat, and not to much CC needed.
Enchanters can charm, so can bard. Bard can mass kite, if thats your thing....
Lots of options
(Way more than what I mentioned here
)