Halo 3 had its flaws as a ranking system. Derankers and boosters. Once you played a lot of games it became impossible to rank up. Everyone knows how ranks increase the playability of the game. So here is my simple system that would to my best knowledge prevent boosting/cheating, Wins!
Rank 1-10
Wins equal rank up.
Three losses equal a derank.
Rank 11-20
Two wins equal rank up.
Two losses equal rank down.
Rank 21-30
Two consecutive wins equal rank up
Two losses equal rank down.
Etc
Quitting/disconnect would count as a lost.
This system with some modification would work in my opinion.