Per Togashi's nen mastery chart, I'll go like this:
Enhancement: Netero and Sacrifice Gon (the only ones in the "ultimate" tier). Uvogin and Komugi are only in the "genius" tier. "Maha"/Zigg, Phinks and Nobunaga are still unknown.
Transmutation: Bisky (the only one in the "ultimate" tier). Hisoka and Youpi are only in the "genius" tier. Killua, Feitan and Machi are still unknown, but Bisky hinted that Killua might easily surpass her someday.
Emission: Meruem and Zeno (the only ones in the "ultimate" tier). Silva is only in the "genius" tier. Razor is merely a "great", doesn't come even close (although he has a scary aura, he's below Knov in nen mastery, for example). Leorio and Franklin are still unknown.
Manipulation: Illumi and Pouf (the only ones in the "genius" tier). Morel incredibly is just a "great" (I think that he makes up with his mindset and base stats). Kalluto, Shalnark and Shoot are still unknown. This is one is the hardest to speculate, as we don't have an "ultimate" yet.
Conjuration: Abengane (incredibly, he's a "ultimate" in Togashi's chart, so it shows that nen mastery isn't related to strength). Kurapika is still an unkown, but he might be up there in the future. Genthru and Kortopi are just in the "genius" tier. Knuckle is an "excellent" and Shizuku is a "great". Kite and Tsubone are still unknown.
Specialists: Alluka hands down (not only he's an "ultimate", but his powers are also just too op). Chrollo is a serious candidate, but he's still in the "genius" tier, the same as Pitou. Tserriednich and Morena are still unknown.