Don Freecs, Ging Freecs, Beyond Netero, Ziggs/Maha Zoldyck, Gyro
Chrollo can become stronger than him if given very powerful Nen Abilities
Also i noticed many Hunter Readers don't know this or missed it somehow but Netero was Member of Pure Paladin Squad
They are said to be comparable to Netero & Legendary Nen Users, so we might see someone or more from that Squad stronger than him