It all depends on what your goals are...

I'm playing a paladin, priest and mage. I'm 71 now and enjoying it. I used a raf tourguide from these forms to 60 and then mainly quests since. I haven't really done instances yet. The goal is that at 80 I can grab some guildies and go do a 5man and be sure that all bases are covered. I plan to raid with them, but I will drop one or two out as needed for the situation.

Paladin gives me mutliboxing easymode tanking. The mage and priest are fun and allow me to AE down quest mobs quickly. I like disc more than holy for the priest to heal and do decent damage. I haven't settled on a spec for the mage yet, but testing arcane atm.

My goals are heavily pve oriented, but I do want to make a 3man arena team to get gear (most likely will just single box in arenas after that). For whatever group you decide on, I highly recommend that you level them all together from level 1. It will let you grow into the group.