Of those I've played, I would say:
1) Halo: Reach
2) Halo: CE
3) Halo 2
4) Halo 3
Reach goes above CE for one reason, and one reason only: No Flood! I've always had a gripe about the Halo series, bragging about it's amazing adaptive enemy AI only to switch it off for half the game whenever the Flood appears to just mob the player. That's always struck me as lazy programming - take the Library in CE for example: up until that point the levels had been varied and interesting, the story had been compelling, and the AI had provided challenging situations to work around. Then, the Flood arrives. Complex level design is replaced with repetition, story is replaced with "Zombies! RUN!", and adaptive AI is replaced with the same old 'head in a straight line towards player regardless of anything' AI that Halo was meant to be saving us from. It just kinda struck me that Bungie were really fired up for the first half of the game, then got bored and decided to clock off early. And as a player, that felt like a slap in the face.
For me the best part of the Halo story has always been the conflict between the UNSC and the Covenant - and of the four Halo games I've played Reach is the only UNSC v Covenant game. I've been told by a few friends that I should play ODST because that game is also Flood-free, but I'm a little dubious about paying full price of a game that was only ever intended to be a stop-gap measure until Reach finished development.
Halo:CE was a brilliant masterpiece as far as I'm concerned (at least up until the Library, anyway), but my dislike of the Flood (or more accurately the implied programming laziness they represented) badly coloured my perception of the rest of the series, until I fell out with the franchise completely with the arrival of 3.
So, yeah. For me, the franchise started on a real high but gradually went downhill until Reach arrived and gave the franchise the send-off it deserved.