I've only played the original and a bit of Double Agent, but have heard nothing but good things about Chaos Theory. Pandora Tomorrow seemingly gets overlooked for some reason. The CIA level is one of the best in my opinion. The final level is quite good as well, as it basically brings everything you've learned and puts it to the test. The level I was stuck at was Kalinatek, which is actually right after the CIA level.
I'm probably going to ask for a number of older Xbox games this Christmas, including Chaos Theory. Hope it's as good as everyone says.
Halo Reach InfoMember since Oct. 2003.