I voted HALO 4. I'm not saying it was a bad campaign, it's just that it had pretty much the lowest replay value to me. After getting all of the achievements(Solo Legendary, Co-op Heroic, terminals, etc.) there has been neither desire nor reason to go back & replay it. And this is the ONLY HALO game I can say that about. Even ODST had some replay value after getting every achievement.
S***, even HALO: CE(despite how incredibly linear it is) has kept me coming back to it quite frequently throughout the years.
RE4, REmake, HALO 1 & 3, & other videos here v http://www.youtube.com/user/Jaidabecca
3 was boring, 2 was boring, CE was good because it was new, 4 was good ish, and Reach was the best.
No way reach was the best, the only thing that kept me going and replaying certain missions was your custom spartan in cut scenes. There were no omg this is awesome moments. But I dont feel it was the most boring. That goes to halo 2.