Siren I have found to be the best. The Hunter's skill tree makes him look good, but up until he gets there he is quite lame. Soldiers are practically meant for multiplayer with all the healing up their arsenal, and Berserkers are pretty much outclassed by most everything else, as you find a good shield and you've fulfilled the role of your Berserker. xD
My skill tree was worked around the Elemental and Controller fields. I find what I have to be great (For those interested, I have full points in Striking, Inner Glow, Hard to Get, Mind Games, Quicksilver, Resilience, Intuition, Phoenix and Silent Resolve). The Dove is godly in this game, so keep it, even if you don't plan on using it.
My one complaint with this game has been that some of the Achievements glitch. I have all but the "25 Corrosive Kills" Achievement because it (Along with the other "25 xxx Kills" ones) can glitch up and not work. Not on first or second playthrough. What makes this most annoying is that Corrosive weapons don't appear until a bit into the game. A minor complaint, but that is one of the only ones I have, which will tell you how much I enjoy this game.