What is the most efficient Custom Challenges?

Im looking for the most efficient Custom Challenges that will help me rank up faster, But won’t hinder the gaming experience. So far I have been choosing…
**1:**Multiplayer matchmaking, Headshots, 125, 3 hours: By choosing this I have been playing loads of swat.
**2:**Multiplayer matchmaking, kills, 145, 3 hours: Also a good choice with swat.
What kind of Custom Challenges do you suggest? Maybe some campaign?

I guess hammer/melee kills with grifball