Who is a difficult jungle to master but when mastered will do wonders?

kayn and leesin

I'm biased, but Kha'Zix.

Rek'Sai is probably your best bet for high skill ceiling, but she's also semi-team-reliant so be careful about that.

Other good choices include Kha'Zix and maybe Lee Sin too.

I'm not even a jungler and I was gonna say Lee Sin 'cause a good one always seems to be amazing.

Right now though Sejuani is seeing some serious light especially in pro play so maybe her, though I don't think she's super difficult so maybe not what you're looking for.

I currently main jg and lee sin for sure I have not even taken the time to try and master him and also kayn

aurelion sol one tricks find success with him some how in the jungle