What is the best way to slap someone hard ?

Upside the head!

With a spanking a machine

Over Your knee Ma’am ❤️

With a leather paddle ......

Use a 2 x 4.

lots of people dont put their whole body into it, you have to aim through the person not at them, just like you throw a ball

With a fish, fresh from the market and whole. Bonus points if it's a tuna!

Shemp Howard (Three Stooges) told fellow actor Christine McIntire to 'let me have it, Chris!" after having to do a number of retakes that were less than effective for the scene.
So she did....unloading a barrage of slaps that were called for....to the point that Shemp was indeed woozy after the shoot.
"Chris, I told you to let me have it, and boy, you did!"

With your hand I guess

Bare bottom

With a hard object or take a big swing.

With a brick

With a dick lol

Make them speeches in front of their favorite crowd.