I won't say "The Great Journey" is one of Halo 2's best efforts, but I thoroughly enjoy it up until the Tartarus fight. I think it puts forth a particular scope and a variety of interesting encounters that aren't seen in other ending levels of any other Halo game.
I also understand the hate for "Halo (3)." Flood-centric levels aren't ever great, and I can see why the scripted nature of the falling plates for the Warthog run rub people the wrong way. I forgive it, though, because the level feels so open. Now, it's not the most open level in Halo 3, but it has its own unique scope – kind of like The Great Journey – and you start off fighting in a snowy field, and then you can jump down and double back around the Control Room. Of course, I never play the level by itself, so this "breath of fresh air" could mostly be a result of spending at least twenty-five minutes in the claustrophobic "Cortana," but that's been my experience. I also think "Halo (3)" starts off beautifully. The combination of Marty's music and the atmosphere from walking through snowy caverns really knocks it out of the park for me.