Fastest way to level up in Modern Warfare 2

Hello,
What is the fastest way to level up in Modern Warfare 2?
You need XP to level up, so should I try finishing all the Challenges? because they give all lot of XP (as many as 10 000)

and what game should i play? free-for-all? that seems like the best one as there are more people to kill and that gives more XP?