1. Halo Reach - Maybe not the most "epic" story, but still well done. Plus it had by far my favorite gameplay. A good challenging campaign, intense Firefight and multiplayer, and the best weapon variety I've seen in any game...ever. I actually liked Armor Abilities too. Plus the armor system was just pure awesome.
2. Halo 2 - No multiplayer, at least not anymore, but I've always been more of a campaign/co-op guy. And this was a great campaign. Tied for Halo 3 for best soundtrack, great story, and the biggest campaign challenge in any Halo game. I still had more fun with Reach on average, so that's why the second game is...second place.
3. Halo 3 - Most people say this game was too easy, but my play style puts it as a good challenge too. Its multiplayer was fun, but (takes cover) I actually prefer Reach's. Oh, and it had an extremely good story.
4. This spot is tied. Between Halo 4...and...Halo CE. Halo 4 had a great story in my opinion but the weapon variety was a joke, I didn't like a lot of the designs, and the campaign was too easy. Enemy AI was also mentally impaired. Halo CE, on the other hand...it may have been the game that started it all, but its atrocious physics, lack of weapons and characters, and poor enemy AI put it low on the list too.
Plus both of these games' multiplayer was boring.
I haven't played ODST, Wars, or Spartan Assault.