I used to do QR with a spec, remote grenade, claymore, MG36, and Warlord. Every match with passes I would get 2-4k exp depending on the server exp boost. Every match took about 10+ minutes with 9 rounds.
Then I used to do HQ Fireteam with passes, a spec, medkits, minigun, warlord, and get about the same amount of exp in 5-10 minutes in Black Market or Bravo with friends. This is how I managed to get from CPT3 to a MAJ 3 in 2 days worth of HQ during one of Nexon's ridiculous 200-400% exp events.
Search and Destroy also gives a lot of EXP assuming you plant and defuse bombs often, and normally I can manage 1-1.5k exp every 9 rounds without passes, but the match time varies.
So HQ is the most boring but efficient exp farm, and then SnD and QR are about tied. It's a matter of preference between the two.