I've been playing Black Ops for awhile now and I continuously end up choosing the same contracts over and over because of my tendency toward certain weapon types and game types. I've been unlikely to select others based upon my experience of just hardly ever completing contracts. Which would be the quickest and easiest to complete?
Getting a party together and doing objective based high reward contracts are the best ones to do. Otherwise just take ones your comfortable doing. Honestly the best way to level fast is to buy all the attachments for weapons you like and do the challeneges like redot dual mags extended mag etc. you'll notice the difference in exp.