Combat Evolved, easily.
First off it has the health, so right away most of the game you are easier to kill than in any other one as the shield and health bar is less health than infinite health. Many levels are limited in guns and they disappear rather quickly, doesn't make for a good combo. The flood can't be destroyed the first time, and will always revive.
The only way Halo 2 is harder is in Co-Op because of that stupid one man dies you both die system. Other than that Halo 2 fixed the flaws that made Combat Evolved difficult.
If I had to order them personally it would go.
1. Combat Evolved
2. Halo 2
3. ODST
4. Halo 3
5. Halo Reach
6. Halo 4
A lot of people think Reach is hard, but you have to play it differently. Once you understand it is quite easy on legendary. I think I beat it in 10 hours.