I personally thought halo 4's campaign was the best part about it, but really? Best campaign by leagues? I'm not trying to be a dick, but that is ridiculous. I think halo 4's campaign is better than halo 3's but Halo: CE and Halo 2 completely blow halo 4's campaign out of the water.
What makes Halo: CE's campaign so good isn't the story on it's own, but how well it's told and how well paced it is. Your goal in Halo CE was simply to save earth, but the twists and turns in the story were pulled off incredibly well. For the first five missions the story wasn't really much to write home about, as your simply trying to wage a guerrilla war on the ring and halt the covenant's progress as much as possible. It's when in the next level, 343 gullity spark, that shit really hits the fan. The whole twist with the flood was so mind blowing and it really made the plot so much more interesting. This beautiful halo in space had a dark secret that not only threatens humanity, but all life in the galaxy. The part in the library when spark was explaining to you what the flood were, then realizing in two betrayals that spark didn't tell you the full story, then finding captain keyes dead in the next mission, and then the final epic destruction of halo in the maw was so fucking well pulled off. It's the twists and turns in the second half of Halo: CE that make it so fucking good, along with the incredibly well paced and varied missions throughout the whole game, and it's why Halo: CE has the best campaign by far.
Halo 2 and Halo reach in my opinion are the only campaigns that even come close to halo: CE. Halo 2 had all this cool shit like when you played as the arbiter and got to see the other side of the conflict, and because of this the story was deeper and more intriguing than halo ce, and the campaign missions, though less sandboxy, were also paced really well, but the campaign fell under the weight of it's own ambition and bungie couldn't finish it, resulting in one of the shittiest cliffhangers in gaming history. Halo 3's campaign wasn't as good because it abandoned a huge amount of the story elements that halo 2 set up for us, such as the arbiter's story.
Halo: reach wasn't so much about the story as it was about the characters. It fucking sucked to see all of your squadmates die, and the ending was pretty epic and sad at the same time, but badass nonetheless.