Runes of Magic US / AU

I'm assuming you're looking at endgame. On Erebos server, definitely tanks.
Mage/warlock and warlock/mage as support are also often sought out and require very little investment in time/gear, and are good classes to start out with while you do runs to get the gear needed for dpsing or tanking.

Knight/warrior is the most commonly used tank combo.

A good dps is always welcome on runs as well, but it is the highest gear requirement of all the options. W/s/wd is probably the best 3 class combo for dps, but you need to max all the classes to get the most out of it so kinda expensive. WD/s is a good entry point to run when you don't quite have your gear up to endgame standards. R/m is a good all around class and not as expensive, only need to max rogue and have mage to 70 tends to do less damage in burns. At least on mithras most groups are physical oriented and going for a magic dps is not really recommended.