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:
Longest individual notation?


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 #
There is actually a very rare 1/342,000,000,000, yeah, lesser chance than any literal lottery… That is… a double disambiguated bishop checkmate- Which will look like this- Ba8xc6# with 7 characters, has been achieved in 0 real life games… It looks like this- here white plays Ba8xc6# next, making it one of the longest possible moves in chess