This is something we saw a lot in the last few days of Beta and I'll give you the constructive answer.
Soloing is a LOT slower that grouping. Luckily, due to the way the archetype system is set up, you can pick up just about anything of an archetype and it can do the job you need it to.
However, if you do want to solo, there is no true 'best' to it. Every class can solo to some degree. At lower levels, priests seem to have an advantage, up until the mid-teens or so. Mages solo pretty well overall, but in all honesty all it takes is a little thought and effort and you can solo with ANY archetype.
I suggest that you play different character types and find one that you like soloing with and can also enjoy in a group.