NOTE: 1-50 Ranking system would be separate from the SR system and would only apply to ranked games.
It seems to me that the most popular, competitive, and comprehensive ranking system Halo has put fourth has been the 1-50 rankings in Halo 2 and Halo 3. This system allowed you to progress in ranks for every 5 levels you achieved. The higher level you were the hgiher skill you had. While the system was basic and I believe could be improved upon, its concept is very good and 343 could add to it.
The system Halo: Reach tried out with the "arena" and ratings weren't very competitive. The ratings system was complicated, confusing, and was never taken nearly as serious as the 1-50 system was.
That is why I believe the 1-50 style system should be brought back. Again, some changes can be made such as having no level cap so that there is always competition, but overall the basic concept should be the same. That is, a system in which you gain levels and ranks based on the games you win and possibly how well you do individually.