It depends of my feeling.
Soldier when you want to be noob (well... some say that im noob all the time
)
Engineer when want to do objectives and collect some XP quite fast
Covert Ops when want use Sten - but in Rons noquarter server you can use it what ever class you are if you just find it
Medic is quite normal class to use. When you just want to play and spend time - no special thinks under working. I don't really use it to selfhealing - i'm so used to play without selfhealing. I love to help others when i'm medic - not myself...
Field ops - well im not really never field... it just not my think.