If your looking to solo primarily for levels / lower end bosses (Im going to define that as Grummus / Klandi / Zlandi etc).. I'd say SK is a good bet.
Clerics are fantastic soloers, but only higher lvls.
Bards aren't bad for soloing their own levels, but definately not capable of lower end bosses.
Monks arent all that bad in either sense, although, they do need to have decent equipment to fair.
Warriors/Zerkers - HAH!
Rogues - HAH!
Mages - Not too bad, just need to know what your doing.
Necros - Great for soloing their own levels, but unless their decently geared, lower end bosses would cause them troubles.
Beastlords - Eh, I suppose they work, but honestly, you're better off going with an SK then a Beastlord.
Shamans- Not all that bad, not what I'd consider my favorite soloing class, but they get the job done.
Druids- If you know how to play them, their one of the most effective soloing classes, even on some of the lower tier bosses.
Think that covered alot of it, correct me if I'm wrong in any of these!
Good hunting