Longest individual notation?

dxc6ep dxc6ep#!! (!! means enable commentary and ep means en passant

So true

exe7(Q)+

I would probably say Qg2xd5# since it is a doubly disambiguated queen capture checkmate and it includes the biggest letters and numbers in the alphabet.
Q, g, d, 5, and 2
example:

Ne3xd5# or doubly disambiguated bishop capture checkmate (Bc6xe4# or Bg8xe6#)

If we're counting less commonly used methods of notation which are still technically following the rules of notation, I propose:

pexd6 e.p. dc #

Lol

Qa4xe8#

Ra1xa2#

Be3xd2#

Ne5xd3#

That's the longest one; any three or more pieces could capture a square, with a checkmate.

e7xd8=Kn# I mean its not wrong to call a knight Kn

A knight is N, not Kn

Yeah in the world we are in

What about dxe1=Q#, like when a pawn promotes for back rank checkmate.

e7xd8=Q#

Well technically it's not but whatever

Nd5xb6# if we are being specific