It depends, I tend to decide what i want to level up that day, so i play that class almost exclusively. Sometimes you can see within a game if tthere is a shortage of one class and fill it.
I like seting up on tripod and laying cover fire, so I have been doing that a lot. Its satisfying to spot and sweep that machine gun around. Even if I don;t get a ton of kills.
Its particualarly effective in the bush people region and the forget to crouch region.
I had a weird flashing session earlier, red and blue whole screen flashes every 2-3 seconds for about 2 games. Terrible. Then it went away all of a sudden.