i like the riot shotgun but i am not sure what is the best ammo for it. what ammo do you use for shotguns?

Depends on the target. Regular 12 gauge for targets with low DT, slugs for higher DT targets or those at a longer range, pulse ammo for robots.

And make sure to get the Shotgun Surgeon + And Stay Back perks.

The GRA add-on gives you other ammo choices, but I haven't used them.