Fave way to do some challenges

For me it's actually playing with friends/BSNers. I don't mind any weapon and kit combo that way because they are too much fun (even though I like to whine at times ). I had some of the most amazing matches thanks to that and I wouldn't change it for anything in the world.
All the Common weapons for example can become fun that way for me. I had matches like Shuriken Drelldepts/Sexbots, Predator TSents, Katana Novaguards/VolSents and they were all fantastic. Same goes for other "weaker" weapons, like Vindicator, Raptor etc.

And then there's also the duo challenge series Hamletic's Tales from the Trenches that currently provides some real fun and interesting setups, like SalInfi/Incisor or DAI/Raptor.