In what tier do you place Hisoka?

[Pocket-God]

Top Tier, High Tier, Mid Tier, Low Tier?

Personally I think he's a High Tier which is pretty much for all the best guys who aren't The King, The Guards or Netero.

Note, I haven't read the manga so I only know up to the end of election arc so I don't know if any other people worth putting in the Top Tier have shown up.

 

[Captain Cadaver]

High tier sounds about right. Beyond those you already mentioned, the only characters that seem capable of defeating him would be Chrollo, Silva, Zeno and some of the stronger Zodiacs like Ging.