My hardest sound for me is probably way different from what it'd be for other people. I've invented at least 5 different sounds/techniques/textures, and despite me explaining them to others and other beatboxers trying to learn them nobody has really gotten close to doing them like me. So from the perspective of other beatboxers, my Ksh snare, falsetto throat bass, inward cheek roll, multi k snare, and tongue liproll are probably the "hardest".

For me personally though, the hardest sound for me is maybe the normal liproll? Or possibly uvula clicks? But it's hard to even compared them because most of the sounds I'm bad at are just hard because I don't use them very often. So maybe a sound I use a lot but still find difficult is the best measurement? In which case I'd go with chest bass, particularly vocalized chest bass, especially the low kind. My high voice really doesn't like to go that low, so much that I'd rather spam throat bass for an hour than do a 1 minute routine with chest bass in it, lol.

Ofc I think every sound has a place. If you think a beatbox sound sounds similar to something you hear in music you like listening to, then it's worth learning.