The ones Netero was probably thinking of
-Himself
-Ging
-Zeno
-Beyond
The last one is hard to say, Netero wouldn't have much knowledge of Chrollo and Chrollo isn't even ranked "ultimate" in Togashi's exhibition as far as proficiency goes with your natural category so that should exclude him from the list.
I think the 5th one could either be Botobai or Pariston if he is really just lying about being weaker. If it's not one of these two then Zzig or an unseen character
I think it's ridiculous for Netero not to know who Chrollo is. The PT are famous and well known throughout everywhere. They constantly steal and probably have killed loads of hunters. Shalnark has taken a Hunter Exam. Silva has fought a PT member. Hes bound to atleast know of him.
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Anyways this question doesnt have anything to do with Netero. I doubt Netero knew who the Top 5 were....just that their was an obvious Top 5 out there.
Being Ultimate just means you've maxed your Nen category. It has nothing to do with PLs. For example Abengane was Ultimate....but he'd get neg-diffed by Chrollo.
Chrollo still has ways to go.