Mine is onyx in slayer and swat
Onyx, but I’m mainly a social player.
Highest I’ve ever gotten was Onyx, probably won’t happen again as I don’t feel like sweating that hard again
Onyx
Diamond 1
Onyx within both SWAT & Slayer playlists. But, admittedly those were earned during the release period of the game when the population was in its peak stages, the skill assessor was relying on TrueSkill version 1 instead of the much more accurate TrueSkill version 2, and prior to all the tuning adjustments that eventually got made to the visual CSR system aimed at preventing runaway rank inflation/deflation, smurfing, tier drop locks, and other such issues from influencing or manipulating a rank from being a more accurate reflection of a person’s actual playlist skill level. So, to me, it doesn’t mean all that much.
As I’m mainly a PvE player, my experience with the PvP part of Halo is extremely limited. Due to these limitations in experience and that I do barely play PvP, the highest rank I have managed reach is Gold V in Team Arena.
Diamond
Champ 8 and onyx1790 in slayer 
Onyx in slayer
Technically my highest was onyx in the 1700’s, but when I got champion it overrode the onyx even though the csr is lower
Champion 
> 2533274792908878;12:
> Champion
I’ll never get Champion, forever Onyx.
> 2533274792908878;12:
> Champion
Uny dropping that subtle flex, I see you 
I hit Champ in 2016 for one game but it never saved on Waypoint apparently . Forever sitting in that Onyx bracket now haha.
Champion 2 in the HWC/HCS FFA playlist! Usually I hover around Champion 20-80 when I commit to playing consistently.
Diamond 4 in SWAT but I stopped playing after 152.