Currently the "best" healer is Warden, because of huge incast single-target heal. Cleric/healer is the second best because of good shields. Summy is third because there are some issues with the class resource (blood). However, in 1-2 weeks (with 8.0.1 update) the best healer is gonna be Summy, Cleric will remain second best and Warden is gonna be nerfed to the last position.
This is only applicable in PvP. In PvE any healer is good enough, in most of the PvE content you don't even need a healer.
Support classes are different. But if to speak mainly about PvE then yes, general function of a support class is to buff their party, dispel/cleanse bufs/debufs and deal damage. Roughly estimating a support class buffing himself produces pretty much the same DPS as a pure DD class that is not being buffed.
There are not many scout tanks around because those playing scouts barely consider a role of tank. This doesn't make scout a bad tank though. I'd say that for Astral at least Scout is the best choice between 3 because of highest DPS in the Aspect of Defense.
Melee scout in the current design is a tank. I.e. if you try to build a melee scout with Aspect of Assault his DPS will be lower than Ranger's and only a bit higher than Scout-tank's. Melee scout isn't viable in PvP as either tank or dps.