With the recent announcement of the pay-to-play questline, it sounds like Regigigas will soon arrive to lord over our Pokemon PC, perhaps to collect dust for most people.
While this thing is quite clearly worthless in PvE, I think it has potential in PvP.
Avalanche + Stone Edge are great charged moves, both being rather cheap, strong and getting great coverage. Avalanche mauls Giratina and friends. Stone Edge hits the Fire types which resist Avalanche. You could throw on Rock Smash to smack around Steels which wall your charged moveset, even if the move itself is far from impressive.
I think it has solid potential in both the Ultra and Master leagues, emphasis on the latter due to it not restricting Gigas’ gargantuan CP