Technician seems to have the most lasting effect on the gameplay,that is you can leave behind and retrieve a sentry gun as well as throw a special freeze grenade that slows xenos down. You can't do that with any other class.
Technician is tops.
BUT Gunner has an extremely powerful and useful Overclock perk which nobody has.
RECON has the lovely pups that makes your FPS chop up but are really fun to use and modify.
Demolition gives us the HEAVY ones which includes the Impact Grenades that basically saves the potatoes. Saving the day is really fun.
Then there are the other ones, DOC and Phalanx.
Doc can give you and the team some buff because Doc is a whore.
And Phalanx, Ive yet to level up to actually use the Shield the way you're intended to. It apparently has a buff that allows you to knock down Xenos.
Am I missing anymore?