I vote AR-15 also: from the perspective of a newcomer to them. I tried a good friend of mine's a couple months ago for the first time.
Until that point I'd shot the following long guns: 410 guage, 12 guage (target loads, buckshot, turkey loads, slugs), .50 cal muzzle loader, Ruger 10-22, CZ452.
I couldn't believe how light the recoil was. There is really no need for a rubber butt-pad. Felt like I was cheating since it didn't hurt. And I thought all those commandos on TV were macho