skirmishers are the weakest class. they don't really offer any form of advantage outside giving everyone more AP to work with, or being able to reach higher ground faster
reapers are able to loving scout out half of the map and all its enemies, and can essentially get like 1 or 2 flank shots in with the shadow mechanic before they are discovered. even then they can go back into concealment and pull off another 1-2 shots. just give their gun like the best laser sight you can find and then they become killing machines
templars are by far the strongest class of all three. they are the ultimate melee class (even better than ranger's ability) because unlike ranger, once you kill someone you can be immune to the next attack, even at like rank 2. its like just as good as the psi operative but has 4x more good powers that can be used