I disagree with Ventius on the Ranger part. In my opinion Rangers are one of the better classes for soloing. Yes they may be slower damage than a rogue or mage, but they can survive a lot better, especially if you invest in Bolas (or Entangle), and most importantly Light Heal. A dex build Ranger could do very well if you run something along the lines of Longshot/Sharp Shot/Bolas/Light Heal with extra points in Rapid Shot (if you don't use haste elixirs). Druids can do well, but they do tend to run into energy issues quickly, unless you are willing to wait between mobs or use energy regen elixirs. Druid skills also take longer to kill mobs because of the focus on DoT, and tend to get interrupted a lot, so that can be frustrating.
My list of classes (based solely on solo leveling capabilities) would be:
(With limited plat)
Ranger (Light Heal means they can heal pretty quickly, decent auto attack damage means that running out of energy isn't horrible, Bolas means they could kite harder mobs).
Warrior (more survivability than Rogues puts them above them imo. Their damage is pretty good if you go with a Rupture build)
Mage (if you have a fire build with a high energy shield and energy boost, you can do surprisingly well).
Druid (they have good survivability and healing, and their damage is good, but slow..running out of energy is a pretty big issue, and energy harvest is pretty trash).
Rogue (their skills are really bursty and high damage, but running out of energy takes that away, their survivability is on par with Rangers, but they don't have a good heal or breather skill).
(With a lot of plat,, let's say combos all the time)
Rogue (with the extra survivability and energy regen/haste from elixirs, their damage potential isn't really limited in any way)
Ranger (close second to Rogues, their skills aren't as bursty, but their auto damage would probably be slightly superior to Rogues, so it's very close)
Warrior (not having to worry about energy regen, plus haste would mean they can do a lot of damage)
Mage (armor/health elixirs would boost survivability, unfortunately because their damage doesn't get a boost, aside from energy regen)
Druid (don't get too much of a benefit from elixirs...though they could drop any support skills and try going for a pure damage build).
And that's my opinion, based on my experience with the classes. Obviously your gear plays a pretty large role in this, and my first list based on absolute minimum plat usage...very limited restos, no elixirs, etc.
I also switched classes around several times while typing this...the classes are all really close in how good they are at soloing, and it mostly comes down to what mobs you're killing, what your gear looks like, your stat build, and which skills you pick.
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