Halo Reach.

I feel it had the best AI and level design, seeing the fall of Reach was awesome, and the final chapter and ending (especially the "Objective: Survive" epilogue) were the finest hour of the whole series in my opinion. The way it tied so beautifully into the beginning of Combat Evolved was brilliant.

As the final Halo game from the original developers it was one hell of a swan song.

Halo 2, followed by Reach and CE

I haven't really finished all of the games in a while, but replaying Halo: Combat Evolved last year, as well as the first few missions of 2, really made me go back and relive memories of these games, as well as experience some of the tracks and levels again. 

Halo: Combat Evolved: 

-Best weapon balancing in campaign (probably, a bit iffy on this point without replaying all of them back to back)

-Tied for second place as the Halo game with the most consistently great level design (though in this case, pretty high highs but very low lows) 

-Very specific charm to this game no other game really has in the series. A very mysterious and wondrous vibe in this game, if I had to put it into words. 

Halo 2:

-Best story in the series

-Best soundtrack in the series (amazing amount of original tunes which manage to be just as iconic as those in the original game, while remixing a few of the tracks from Combat Evolved and putting them in their best form (looking at you, "Heretic, Hero"))

Halo 3:

-Most consistent and best level design overall (too bad it has the worst stage in Halo history) 

-Best arsenal of weapons and abilities (equipment)  

-Tied for best artstyle 

Halo: Reach: 

-Best weapon balancing in multiplayer (it's really too bad it's such an unbalanced multiplayer mode overall) 

-Tied for best artstyle (Really dig the semi-realistic and gritty texture work complimented with the high contrast of the dark colors with the saturated ones, closest thing to it that I know of is Xenoblade X oddly enough)

-Tied for second place as the Halo game with the most consistently great level design (maybe few levels are quite as memorable as Combat Evolved's peaks, but nothing here is glaringly mediocre either unlike in that game)

-Edit: Best A.I. in the series BY FAR, can't believe I forgot to write this when I literally planned to while thinking of writing in here lol. Thanks @curl-6 for reminding me with your comment

Keeping all of this in mind, I'd have to say Halo 3 is my favorite. The way that game creates set pieces and makes them into actual gameplay is honestly a little mindblowing. 

Edit: The rest of the games aren't really worth talking about, lol. 

Halo 2/3 were my favorite. Hard to pick one.

I have pretty high hopes for Halo Infinite. Based on everything we have seen and heard, it's looking to be an excellent Halo game. The performance and graphical issues should certainly be fixed with a Fall 2021 release date.

ODST

Halo 3 by a long shot.
Really gave the Xbox 360 the needed AAA game to overturn the PR nightmare that was RROD and shift consoles.

Had a fantastic campaign that was fun to play with friends and replay to unlock content... And the Multiplayer with forge was just endless fun.
And visually it was actually a very nice looking game on release unlike Halo 5.

I would argue ODST is Halo 3's expansion as ODST came with the Halo 3 multiplayer disc and used the same game engine etc'.

Halo 3 > Halo CE > Halo 2 > Halo Reach > Halo Wars > Halo Wars 2 > Halo 4 > Halo Spartan Assault > Halo Spartan Strike > Halo 5 > Halo Master Chief Collection (As it was a mess on launch.)

Halo: Combat Evolved for me! Countless memories with friends.

Halo 2...... but I find myself slowly seeing Halo 3 as the superior game (mainly for the MP).

I. Fucking. Told. You. 

halo 3, but I would also rate all the Bungie Halos highly.