I too am going to say Support. Personally, I have never, NEVER, used a Support for the Reaction Shot skills they have. Mine are always pure medics.
The way I see this question is "What class would be best on it's own?", sort of like "What would you rather have team of?" Having a lone Support allows it to move far ('cause screw Smoke and Mirrors), deal decent damage and, if it does get hit, heal itself for 10 health (with the Foundry upgrade. Without, that goes back to 6, which is still pretty decent). Moreover, if it finds itself stuck dealing with 5 Mutons at once (or something like that) then it can just drop a smoke behind full cover, and it almost guarantees every shot to miss. Especially with Ghost armor.
I'm getting far to in-depth with this.
Point is, a single medic-build Support can revive 30 health on itself, make pretty much everything miss, and run the hell away if necessary. Offensively, it doesn't have the same capabilities, but I play defensively, so it suits me.
BTW Psionic Support is one of the most OP things in the game.