By the power of truth, I, while living, have conquered the universe.
I consider Halo CE, 2 and ODST to be the best of the Halo games in terms of campaign. They're unique, stunning in just about every way and present an interesting story.
Yes, Halo 2 had its flaws, but so does every game. In the end it was one of the best XBL games ever, whilst CE was also brilliant and gave me many great memories of LAN parties, it was just as unbalanced as any other game with the 3-shot Pistol and the fact you could kill anyone by simply tapping their little toe with a vehicle.