The best exp:
If you're god (yes ONE o) is Search and Destroy, I get an avarage of 20kills per match (the same as i do in a TDM

) plus the match bonus I get about 1,000 exp per match per couple of minutes if I rush and we're on the attacking team
If you're good is hardcore Search and Destroy, it's easier to get a kill so more fluke kills happen, and you can just camp the whole match with a geweher silenced and get a decent about of exp
If you're decent is headquarters, 5 exp if you have captured it for every 2 or 3 seconds you have it captured, if you are alive for most of the game and get very few deaths you get massive match bonus and you don't need to get any kills to get exp
If you're bad is sabotage, you can just be a sniper camping in a building with betties watching your bomb site and you will get some kills, or you can go risky and try planting the bomb
If you're bloody awful and don't deserve to be playing this game how bad you are, do rank up matches.... they're really slow and boring and never work as well as you imagine, some retard always come up and kills someone or you spend ages trying to get into one, then the host leaves or something like that, or someone captures A instead of just B