Forgive me if I am repeating anything that has already been posted or is something you have seen a million times elsewhere and is in your mind useless information, but I refuse (at 3:12am) to read through seven pages of posts...so here's my idea.
Personally I have always preferred the Halo 3 ranking system for several reasons; Firstly because I liked how it drove players to be more competitive and always made me want to play harder and better than before...being a rather competitive player little matters more than the drive to compete. Secondly the system was solid and you felt...something of a sense of accomplishment for having worked hard to reach a new "tier" in the system. Lastly because I never had the chance to properly play Halo 2 online...so I've never known anything better.
So my recommendation is this. Why not have it similar to the halo 3, in the sense that you had that 1-50 ranking system for the competitive types but also had that relaxed social version to each gametype to cater to those who enjoy playing those same gametypes ..but in a less competitive environment within the same playlists..and of course that playlist off to the side to accommodate those who like the more 'far out' games...though I have always said the one thing I never liked about the halo 3 system is that if you reached a level...then lost...you would lose that level and have to fight harder just to retrace your steps... now I have heard the arguments for this part of the system...but it is off putting and not always fair...there are a number of things in-game that could cause a person to lose the game... e.g. connection issues or (often in the case of halo 3) host booting, things like these whereby through no fault of the player their rank is decreased, and while you may argue that "if you really deserve it then....." I'm sure many of you know this...so I propose this...should this kind of system be used why not have it so that when you reach a level you do not go down if you lose...rather you maintain that level but it is only slightly difficult to level up...this way you don't get bs'd into a lesser rank and if you really deserve to level up...then you inevitably will.
Also...the one xp per game was also a huge bore...why not make it a bit different...like...xp given depending on performance....depending on something like Kills...a rating or something...or even K/D ratio
0-10 Kills = 1xp
11-14 kills = 2xp
15-18 Kills = 2xp
18-20 Kills = 3xp
20+ Kills = 4xp
This idea might seem kind of lame...but I just wanted to contribute something to the discussion.
Anyway It seems though that from what I have read of most peoples posts...the older ranking systems are preferred...and I agree.