One of the oldest must be VFA-14 Tophatters, who turned 80 several
years ago.

You could seek for the largest among Fleet Replacement Squadrons, for
example those operating F/A-18 (AFAIK up to 40 a/c each). I do not know
what are the PAA (Primary Aircraft Assigned) numbers for VT/HT training
squadrons.

Best regards,

Jacek