I like Halo3’s the most. Having the Katana and 1000Gamerscore unlocked just actually meant something back then - along with achieving Recon armor on Bungie day by having my team play 24 hours straight and being awarded Recon for having most stickies in the world on that day.
Achilles H5 Company grind was fun as well for getting that as world’s first but Recon still had more prestige rewarding feel to it.
I would love to see halo 4’s style return, you knew what you were gonna get and it wasn’t RNG based like it is now, there were some things about the packs though like the classic helmets or if you want to get armor early you can buy packs or you can just go through the grind to get it.
You should earn EXP like in Reach but there also should be the Rank system of Halo 3.
You should unlock Armor with cR, some armor just at a special level, some armor with achievements only, like the Recon in Halo 3 with the Vidmaster Challenges.
Also they should bring back the military ranks from Halo 3 - and really should stop with that - Yoink! - stuff of numbers. I prefer it to see the 5-Star-General and the Emblem in my career than a 152 or a 130. Numbers are worse.
The Achilles grind was kinda good but ruined WarZone for everyone who wasn’t really good and not with a team. [I know they fixed it so that full teams have to play against other full teams but that came really slow]
And - in the name of god - never ever those stupid REQ-Packs again!