Playing nowadays with my experience, I don't find any of them hard anymore. So basing this off initial playthroughs, I'm going to say Unova 2's league was really easy, even the champion. I went through without losing anyone my first time playing Black 2, and I couldn't even read the game. Then again, it only came out last year and I was 20, so I knew what I was doing at least.
Emerald (not RS) was pretty easy too. They gave you access to a level 70 Rayquaza, so if you really wanted to, you could blaze through there. ~_~ I only brought it with me once, and even still, I kept Ray in reserve really. Didn't need to use him but one time, if memory serves. lol