Never played H2 or ODST. But here it goes, from easiest to hardest. (Based on Legendary observations)
Halo 4 - Retarded enemy AI, the enemies never focus fire or continuously fire, they're slow-moving, and there's ammo EVERYWHERE. Plus the aim assist. The range you're at doesn't matter very much when your bullets are magnetic. Also, Prometheans are so predictable, whenever they sword rush me I've already moved three feet away from their attack. I mean, it only happens when they've been hit a lot, and most of the time, they sort of bow and twist to the right before they rush you.
Halo CE - The game is hard the first time on each difficulty. Well, haven't tried Easy ever, but whatever. But after that it's not very hard. Sure, the hitboxes were an embarrassment, and the weapon selection was limited, but overall it isn't that hard. But there were moments that were frustrating, like BANSHEES or the huge number of SpecOps Elites on Keyes. The Library, though, I don't understand. It's no harder than the average CE level. It's just longer.
Halo 3 - This one's really fun. But the enemy AI's huge improvements and an entire host of new ways to die make it hard regardless. Anything involving Wraiths (Not AA) was extremely frustrating, and the enemies could shoot through your vehicle plating with almost no damage dampening. And yet the vehicles themselves were almost indestructible.
Halo Reach - The first level of Halo 3 is harder than the first level of Halo Reach, but take it any further than that and Reach is harder. Invincible Hunters, hordes of smart enemies with high health, lots of well-placed explosions, and the fact that if you step anywhere you will be met with focused and continuous fire. Enemies could dual-wield, but you couldn't. And the Sabre flight...if you're shot, get out of the way immediately. Unlike how you can fight for a second or two while being shot on foot, if you have a Seraph on your tail and wait even a second before moving out of the way, you will already be dead when you reach the cover.
Thankfully, Halo Reach is so dynamic and fun that I enjoy all of that.