Well, if you go for dedicated classes you will do that dedicated role:
warrior . pve tank, pvp-dps works nice. you might even dps in raids if you are lucky.
priest and paladin. heal. both in pve and pvp. In pvp you might have to put a lot of points in disciple build and not be so good at pve. with paladin you might do a very nice job at tanking 5-man heroics or kara. even some raids.
rogue,hunter - dps. that's about it.
shaman,druid - now this is the fun. druids can tank, heal and dps in pve. in pvp you might be pushed in the healer role. shamans dps and heal. in raids you might have to choose between elementar or rest.
bottom line there is no "best class" because that would be the only class.
I wouldn't say there is a best class. If I had to choose though, I would go with druids. They can tank, dps or heal. Pallies can do the same, but I think druids are more varied- plus they can stealth. --- Sanity is just a lack of understanding. At least, that's what the voices tell me. (ER WoT) ™
but having dranei warrior means i can tank better which this class wins with healing spell.
Unless oyu have any +healing, the heal is very unimpressive at level 70, in addition to this, the heal has a cast time, which means that while casting it you have a 0% chance to dodge, block and parry, at end game this'll result in you taking more damage than the heal would heal you for. --- Not changing this sig till I find a better one, started July 6, 2005.