The game has been updated so that most classes can solo very well. It was not really so in the begining, though.
If you REALLY want to focus on solo'ing, I would do the classic EQ-style move, and go pet-class, such as Conjuror, or Necro. These solo very well, from what I understand :)
But, if you want to do mixed solo/some partying, then you could pick anything, really. Pretty much any class that has some healing will do amazing as a solo'r.