Halo 2. The game was rushed during development, and as a result it was not properly tested on Legendary, which led to... well... I think we all know by now.
It's well known that Halo 2 was rushed, but not properly tested in Legendary? I wouldn't take that claim at face value.
Halo CE is the easiest, because the combat is slower paced due to the original Halo having been planned and initially developed as an RTS for the Mac before Microsoft purchased Bungie and changed their direction.
This is not accurate. Halo was already a shooter by the 1999 Macworld trailer, and it had started morphing into that direction by the summer of 1998
, two years before Microsoft announced the acquisition of Bungie. I don't believe the pace of the campaign has anything to do with the early history of the game. There are probably a number of reasons why the CE campaign is slow from the preferences of the creators to the setting of the game and technical limitations.