Here is a listing of the kegs by manufacturer and the threads associated with those kegs:
Firestone V Challenger, Firestone VI Challenger, Firestone Super Challenger
Gas 9/16"-18 threads per inch
Liquid 5/8" -18 threads per inch
Cornelius Spartan & Super Champion (Most Popular)
Gas 19/32" - 18 threads per inch
Liquid 19/32" - 18 threads per inch
Cornelius R (Pin Lock) (Most Popular)
Gas 19/32" - 18 threads per inch (2-pin)
Liquid 19/32" - 18 threads per inch (3-pin)
John Wood 85, Firestone Challenger
Gas 11/16" - 18 threads per inch
Liquid 3/4" - 18 threads per inch
Firestone A, Firestone R, John Wood RA, John Wood RC (pin lock)
Gas 9/16" - 18 threads per inch (2-pin)
Liquid 9/16" - 18 threads per inch (3-pin)