1) Druids... Why? Cause more then one is needed for most bosses.
2) warrior... only one is needed to take high end bosses
3) rangers... there support an great solo ability is amzing for killing adds and helping I control boss
4) mages... they have nice support to help group with dps can be decent with adds and can have very high dps themself
5) rogues... Possibly least usefull. Not great with adds if there are many and just help kill boss faster (more of a second iPhone class and leave it on auto boss)
+10
A lot of people say mages arent important, that their value is undermined because rogue and ranger dominate the DPS area. This is not true. Mages have important lures that in an endgame bossfight make an immense difference by decreasing boss resistance. Thank you high level mages! xD
Rangers do have a high survivability rate and therefore I believe are second best support classes (Aside from of course druids who are built for healing. for rangers its all pretty much all due to light heal though

).
Rogues do have some nice nuke spells (nuking - terminology definition - a nuke spell is a spell that deals a high amount of damage at one time) in this case Sneaky Attack and Assasinate. Overall, this is the class that is the real not-so-useful boss class, not mages. Their DPS skills are easily replaced by that of other classes.
Warrior and Druid, obviously these are THE best support/buff/healing and tank classes in the game so far. Please correct me if I am wrong but I wouldnt be surprised if warriors had a higher vitality:health ratio than other classes. They also have Protective stance, a skill vital as well as extremely useful for tanking, as it increases armor value, and of course taunt which manages enemy aggro. Druids are decked with healing spells (as I am sure everyone knows) and are excellent buffing classes too (buffing class - a class like shaman if you have ever played WoW - a buff is a skill that can be cased on yourself or an ally to increase stats that are beneficial to the user i.e. armor, health, health regen, energy regen, etc).