Me personally? 6, but I don't know if I'd say it's exactly worse than the others, because it's a great game; but it's because I just don't like the style 4 and 5 had compared to Combat Evolved through Reach. I prefer the slower more methodical pace of the earlier ones.
If I had to rate them:
3
Reach
2
Combat Evolved
4
5
Note that this is someone who didnt have Gold on his original Xbox, so I never experienced much of their multiplayer outside of Master Chief Collection.