Definitely ODST by far. I know that game got a lot of hate, but it will probably always remain as my favorite campaign. The whole thing with being able to roam around new mombasa looking for audiophiles or engineers and finding little secrets here and there. It was a freedom that you can't experience in any other Halo campaign.
Not including Wars due to the fact that I never finished it. I liked the campaign but I can't honestly list it fairly. Also, despite the fact that I put Reach at number 5 doesn't mean I didn't like it, in fact, I loved it but not as much as I did the games before it. I like 4's campaign but there is pretty much no replay value, imo, since after getting the achievements, commendations, & terminals there's nothing to do in it. Well, maybe SLASO some day but until then the campaign will remain untouched.
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Reach was a great campaign. The gameplay was varied and did a good job of mixing enemy groups and vehicle combat, the characters were unique and I loved being able to customize my Spartan.
ODST had a great squad with awesome voice actors, and I loved the noir styling. The various levels seperated different gameplay styles well, and I actually liked exploring the city.
4 is my favorite of the main Halo's, Cortana and Chief finally got a personality and I liked the new enemies.
2 is probably one of my most played campaigns, but the difficulty was frustrating and the ending is horrible.
3 I felt was pretty forgetable, the story was dull and the flood levels were terrible.
I hate CE. I never understood the love for this campaign, it's repetative, the autosaves are terrible, and the flood levels are the WORST. And I say this as someone who played it first, way back in 2001. Never gonna understand the love for this one, I thought it was bad then and it's aged even worse IMO.
The big brain am winning again! I am the greetest! Now I am leaving Earth for no raisin!
Halo 2 is my favorite of them all, but I think technically speaking I would have to say CE has the best one. Halo 3's campaign is brilliant as long as you do Co-op. Going through it by yourself sucks. Reach and 4 were just weak.
I gotta try ODST one of these days.
Ah, yes, the Negotiator: General Kenobi <sneaky beeping>