Generally speaking, leveling in a group of three is always going to be dramatically faster than trying to go it alone, even with the "best" classes for going it alone. (And two will be faster than one as well, but three is really where it's at.)

Probably the most common way to deal with this is to make something that is desired by groups; another option is to let go of the idea of leveling very fast and just kind of do your own thing.

High-demand group characters tend to include the really top-tier tanks (shield paladin, defense shifter past the low levels, defensive warrior spec such as sword/spear/pole, etc.) or better support characters (healer, transmuter, bard, etc.)

From a purely mechanical perspective, I'm almost always going to have a better groupmate option around than a thief -- but I'm likely to help out thieves in my cabal/religion/etc. if I can.