Isaac Netero states that he was the former top Nen user in the world. Ging Freecss is one of the current Top 5 Nen users. The other 4 are currently unknown.
Another anwser: Meruem and his royal guard are confirmed to be even stronger than Netero, so the top nen users in the world (that we know of) in order are those:
- Isaac Netero
- Ging Freecss (out of order, his nen was never shown)