Just play whatever you like the most. you don't even really need to have a main. Being good at every class is ideal anyway, but if you really think you need one class to focus on just think about what your strengths are.
Good leader + Good DM = Pocket
good gamesense + movement + comms = medic
Good DM + offclassing = Scout
good at being a fucking %^$###@$*%#$% ^#@$(@#$ + offclassing + rocket jumping = roamer
good prediction + positioning + comms = demo.
Before anybody says "oh but capnfapn good positioning is important as every class! UR BAD", I'm just saying that different skills are more important for certain classes than others.