Call of Duty 4: Modern Warfare/Elite

Elite Challenges are very hard to get, as you need to be level 51 to get them, you will complete most of these for a complete list of challenges rather than for the XP. If you are going to do Prestige mode (Xbox 360 and PS3), you might as well skip out some of these until you finally do these as there will be no recognition that you did these and will have to do them again.