Heres a good order of difficulty of skill leve/class usage:
From easiest to hardest to be good with...
1. Slipgate
2. Doom
3. Arena
4. Earth
5. Strogg
I don't - not completely.
From easiest to hardest score a kill with...
1. Doom
2. Slipgate
3. Earth
4. Arena5. Strogg
Also, don't forget that class strength is always dependant upon the map being played.