@iphonetrabaohanh After 3 years of Crossfit, the two movements that elude me are pistols (I'm oh so close, more I think a mobility thing than anything else), and handstand walking/holds. Albeit it for both, neither are things that come up on a regular basis, the amount of hours practicing them both, or working on them as weaknesses just isn't there.
I'd put clean and jerks, and snatches up there as difficult to master, neither are the hardest to learn, but some of the little nuances can make all the difference for them. Ring and Bar muscles ups are in sort of the same category, if you have the strength, and the body proprioception, they aren't the hardest to get, but stringing multiples together require a higher mastery than just throwing yourself really hard at the first one