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Well, I've gotten every single weapon maxed and diamond, so I feel qualified to answer this question:
Use something different every game. Just try something crazy. I went 34-12 the other day using a Skorpion with Rapid Fire and a Target Finder. Give a silenced Executioner a go. Experiment with Overkill loadouts. Put on two flashbangs and two stun grenades. Use a Five-Seven and six perks.
If it's performance you want, can't go wrong with red dot/stock/quickdraw M8A1 or FAL; or a long barrel/quickdraw MSMC.
Alternatively, get to gilding the M1216. It's pretty fun to use.
Alternatively alternatively, use this: http://bo2rand.xmajikx.com/ Random class generator. Gives some pretty interesting combos.