Man, that really isn't an easy question to answer. I mean I have strong points for each Halo game in and of themselves.
I mean Halo: Combat Evolved was just... epic, like a huge game and my first real time I've allowed myself to become lost in a virtual world. It had a great sci fi atmosphere and some great action and gameplay and the plot was intriguing and greatly put together. I loved that game. Action was heavy, there was a level of intrigue, the music blew me away and that final warthog drive still gives me excited chills to this day.
Playing this game, actually makes me feel like I'm alive.
Then came Halo 2. I just remember constantly playing the level 'Outskirts' and then 'Metropolis' over and over again co-operatively at a friends house, but then the point that just drove me to the level of insane fandom was what happened after the cutscene where Masterchief blew up that Scarab.
The Arbiter came into the plot, and let me tell you something, the experience exploded into greatness for me then. It was without question one of the most interesting ideas in the game series to date, taking the side of the Covenant gave the whole conflict a new shade of depth and while plenty of people with anger management issues and difficulty spelling words without adding numbers to them took issue with this, I thought it was great.
Now then comes Halo 3, after the greatest cliffhanger in gaming history, Halo 3 finally arrives. And to be honest, there are a few faults, the campaign was quite short and the Arbiter had been pushed entirely to the side and the plot seemed determined to just find a way to resolve all the dangling plot threads rather than try to elaborate on them first.
That said, it still felt pretty epic.
However, the multiplayer was still great fun to play with friends and the visuals were great and there were still plenty of new ideas for gameplay, including the new 'Forge' and 'Theatre' modes.
So I say it was ultimatley a good game.
I liked ODST, it wasn't amazing but it had a nice spin on things and showed the Covenant invasion from a different angle. Plus it had the cast of Firefly doing the voices, and a nice jazz soundtrack.
Finally, there is Reach. I have already written a very big review [http://www.escapistmagazine.com/forums/read/326.233551-Vrex-behind-the-controller-Halo-Reach] on the matter and I don't think I should need to repeat myself. But still let it be clear, Reach is EPIC.
So, having difficulty picking I'll say Halo 2 had the greatest story, Halo: Combat Evolved had the greatest single player, Halo 3 had the greatest feeling of epicness, ODST had a very interesting new way of looking at things and Reach had the best.... everything else.