My ranking, from best to worst:
1) Halo CE. I love the sheer scale of the levels, something that still hasn't been properly replicated in a Halo game since (though Infinite looks to finally be remedying this). The combat loop is also the most satisfying in the series thanks largely to the challenging but fair difficulty balancing. Even on Legendary, it allows you to experiment and try different strategies instead of pressuring you to engage in a narrow selection of tactics. It's amazing how you can make for tough yet fun encounters by focusing on things other than making it to where a single enemy can kill you in half a second or less with a regular gun.
2) Reach. Though the story wasn't anything to write home about (plus it retcons established plot points from the novels), the enemy AI was improved from Halo 2 & 3, as was the combat & level design.
3) ODST. Shorter than the other games, but improves on a lot of Halo 3's shortcomings. The new musical direction and the cool aesthetic of New Mombasa at night were nice touches as well.
4) Halo 2 & Halo 3 (tie). Halo 2 saw a big step up in terms of storytelling, with the Arbiter's arc being quite enjoyable, but the level designs were far more linear than in CE, the Flood missions were an aggravation, there was that awful cliffhanger, and the difficulty was ridiculously punishing compared to CE, with the damage scaling being passed on to future Halo games. In Halo 3, the Brutes are less fun to fight than Elites, the AI took a step down from previous games, and the first four missions are mostly pointless filler, but the levels are better designed and more consistent in quality than in Halo 2 and the difficulty isn't quite as punishing despite the damage scaling being the same (lackluster enemy AI, lower enemy health, and toned down Sniper Jackals probably helped).
5) Halo 5. The level design was a huge improvement over Halo 4, and the Prometheans aren't as big of a pain in the ass as in Halo 4... except the repetitive Warden boss battles. The squad mechanics seemed interesting at first, but, as per normal, friendly AI in Halo isn't good at all, not to mention the difficulty has been cranked up to compensate, resulting in you getting quickly downed Gears-style, having to wait for your peabrained AI squad mates to show up and revive you. The story is serviceable, but that's it.
6) Halo 4. While the Chief-Cortana dynamic was the highlight of this game and was done very well, the level designs are perhaps the most linear in the series, the Prometheans aren't fun to fight, and it was just a boring campaign in general. I rarely feel like replaying this one.