Anything which demands some attention and is hard to perfect.
For example i like beeing a Mastermind in CoV. you have to command up to six henchmen, monitor the group, debuff key foes, maybe heal a bit, buff your guys and henchmen. Do some damage, switch to tank with bodyguard mode, switch to offensive to take aggro...
Other classes are just boring with the: I hit it until it drops. Or: I heal the tank, till the enemy drops...
If there is no such class i tend to get something with tactical value... or sometimes just a guy with a huge sword till i am bored (very soon after that)