This is constantly rebalanced so what is said today might be wrong tomorrow. I recommend JGOD on YT. He keeps the world up to date on the best weapons and strats.
As for whether M4 or Kilo is best. The Kilo was META the last couple of months or so. It is a looong time since M4 was best. Its still decent though but a lot of other wepaons might be more viable.