1. Halo CE - It started it all, and had the most INTENSE campaign ever, and the levels were huge!
2. Halo 3 - It improved upon what was already great with Halo 2. It added equipment which added more variety to the game, it was the first game with Forge mode and armour customization (i haz reconz!!!11one!!!eleven111) and concluded the war with the Covenant, and the great trilogy.
3. Halo 2 - It had amazing multiplayer modes, and added lots of new features that Halo doesn't seem the same without, like hijacking, duel-wielding, 1-50 skill-based ranking system, and playable Sangheli (aka Elites)
4. Halo 4 - Whilst it wasn't the best in the series, 343i's first entry to the series wasn't a half-bad one. It added some features, both good and bad but it took away some of the awesome features from Halo 2 and 3, like duel-wielding and in-game 1-50 skill-based ranking system. But it added the Mantis, custom loadouts (which I actually like, unlike most people), Forge features (magnets, duplication and highlighting, so you don't accidentally move the wrong pieces), Sprint (which is mostly disliked and I see why). It was a decent game, but it lacked the replay value of Halo 2 or 3.
5. Halo 3: ODST - This game would've been much higher on the list if it had more features and its own engine. Firefight mode makes its début, and you play as multiple characters, much like in Halo 2 (John-117 and Thel 'Vadam). But it falls short by having no original multiplayer mode, and a campaign that's shorter than you'd want it.
6. Halo: Reach - I feel like I'm the only one on this forum who isn't a huge fan of this game. It has some terrible weapons (The Assault Rifle wasn't nearly as good as the ones found in previous games, the Plasma Repeater had way too much recoil, and give me the Covie Carbine over the Needle Rifle any day), less challenging Firefight mode (Armour Abilities made it too easy, and I also disliked most of the maps), the maps were either dull Forge maps (Dull, because of the lack of colour; They're mostly grey.) or maps recycled from the campaign, same with Firefight. The Bloom made the game completely random at times and sometimes, people would win DMR fights by spamming the trigger, and getting lucky with their 5-shot kill. But I did like playing Custom Games and Forging. Some parts of the campaign were fun too, like the space combat but for the most part, the campaign was dull and depressing. Reach isn't a terrible game, it was just kinda disappointing compared to previous Halo games.