If there was a 'most powerful' class of any kind, everyone would play it and you can imagine how fun that would be. (I can root and dot! Really? So can I.)
Anyway, I feel any class can solo. But there are those that should and those that should not. Example would be a Monk or Rogue. They can solo but they are best 'suited' to be joined with a tank of some sort so that they can do what they are designed to do (Rogue = backstab).
Druids and necromancers used to be the considered the best for soloing. Beastlords were added to the list of solo capable classes cause of their pet and ability to heal it. Mages are considered by many to be an effective solo class for the same reason as a beastlord. Shamans IMO are underrated as far as their ability to solo. Clerics can solo undead VERY well. Paladins and Shadow Knights also have great ability to solo (Pally can heal him/her self, SKs have pets and life taps). Bards are incredible soloist (no pun intended). They can slow mobs, speed their movement and Area Effect damage the mobs that are following. (Aggro everything and run in circles until they die). Question really is, What do you want to do?
I agree with the overall opinion of Visagoth but in the same respect, you have ALOT of options. Just chose the one that best suits your style. Like nature? Pick a druid/Ranger. Like Death? Pick a Necro/SK. Like putting ice packs on your fingers? Play a bard.
I know there are those that disagree but that's why they call it 'opinion'.