What is the most powerful anti tank rifle of all time that is man portable and not a recoilless rifle also it does not matter if the anti tank rifle is not in military service?

Answer 1

Of the rifles actually dubbed as 'anti-tank' rifles, it would have been the Boys .55 inch rifle. Although many of them were rechambered for the 12.7x99 (.50 BMG) cartridge in the 80s, when the concept was revived in the form of heavy/anti-materiel sniping, the .55 Boys cartridge was considerably more powerful than the BMG cartridge.

Of heavy sniping/anti-materiel rifles, the most powerful would currently be the South African Denel NTW-20x110 and Croatian RT-20, which fire a 20x110 cannon cartridge originally intended for use in the Hispano-Suiza aircraft cannons.

When you say "man portable", I'm assuming that you're referring to systems which can be transported and operated by an individual. Weapons such as the 37mm anti-tank guns of early WWII could possibly be wheeled around by an individual for short durations, but they were crew served weapons.

The world's most powerful anti-tank rifle is Japanese type 97 20mm.