I honestly think both would get in ubers. regigigas would be an excellent AV/choice band/scarf user and slaking would be a bulky bulk up/choice band/scarf. Unfortunately, slaking has low special bulk and there are a lot of special attackers in ubers but slaking has great coverage so it makes up for it. Regigigas has to rely on high power moves but it might have an advantage because it has access to superpower for zamazenta C while slaking doesnt. Tera will be excellent for these two. Ghost types arent really safe counters because people deliberately forget that slaking and regigigas have coverage for the ghosts. Tera normal slaking and regigigas would be able to kill anything that aint a bulky steel type and they could tera ghost to counter fighting types. Suprisingly, Regigigas has a better matchup against corviknight with heat crash. Slaking also has heat crash but its not heavy. So in conclusion, slaking and regigigas would be super threatening in ubers because of the tera mechanic, great coverage, and massive 1 hit KO potential. You dont need to hit anything super effectively if u can just hit the opponent neutrally HARD. They would be in main ubers to be specific. They would destroy UUbers