Getting close on my third, Kill Confirmed since that one only has 3 challenges to complete. It's taking longer than I hoped though since I suck at 'kill' focused gamemodes, lol.
After I get KC, I'll probably try Demo or Hardpoint. I doubt I'll ever get a 20 killstreak in TDM so I'll probably never get that one complete. I wish multi team had it's own set of challenges as that mode is actually pretty fun from what I've played of it. The party games, like KC, only have a few challenges as well so those might be easier to get too.