Slaking by a mile
They both have trash abilities, but Regigigas is worse. IT RESTARTS SLOW START WHEN SWITCHING IN and if you survive slowstart somehow, no healing while slaking gets Slack off
Slaking can be on the field for 3 160 attacking moves while regigigas has 5 trash turns
as demon flygawne said, Giga impact fits slakings ability while slow start cant be that good.
skill swap/gastro acid is very situational and these 2 deserve to be in the tier they are in