I have thrown double fulls, and Arabian through to fulls. Which of those is harder I'm not sure, they seem about par to me. The hardest skill for me personally... Dive rolls. Still can't seem to get them right though I also rarely practice them. And I've done a back walkover before, but that was also very difficult.
The easiest for me was a back handspring. I had it down in a couple of hours of lessons.
my kid is only level 3, but she would say FWO. She HATES them with an undying passion that knows no bounds. She spent 18 months trying to get that skill.
I skipped back walkover and worked on back hand springs during a one day-per-week-8-week tumbling class session. Stopped going for 6 about months. Then when I joined allstar cheer in October of 2015 of the same year as I did the tumbling class session, I worked on my back hand spring that whole season. Got it like a week or 2 after the season ended. I've been working on my punch front thru to layout this whole season. So its a tie between those 2. The easiest skill was either punch front or standing tuck