For gen 8 Ubers, the most common type (excluding Gigantamaxers) is Steel, which has 8 Pokemon (Magearna, Melmetal, Zacian, Zacian-C, Zamazenta, Zamazenta-C, Solgaleo, and Necrozma-Dusk Mane), edging out Dragon, which has 6 Pokemon, and Psychic, which has 5. Steel has many Pokemon in the tier because most are bulky and have a very large offensive fresence in the metagame, especially Zacian-Crowned
In OU, steel, again, has the most Pokemon in the tier, at 8 (Jirachi, Magnezone, Corviknight, Scizor, Skarmory, Aegislash, Ferrothorn, and Excadrill), largely due to the type's overall versatility, being able to set up rocks and spikes(ferrothron, Excadrill, Skarmory), trap other steels(Magnozone), revenge-kill(Scizor, scarf Jirachi), endlessly flinch(Jirachi), and remove hazards(Skarmory, Corviknight, Excadrill).
As for UUBL, Poison, Dragon, Bug, and Ice are tied. The UU banlist is rather small.
In UU, the most common type is Water. The reason for this is because Water is a pretty good defensive typing, and most water types offer support options, like spinning (Starmie, Tentacruel) , Trick room support (Slowking, Slowbro), status and bulk(Rotom Wash, Araquanid), entry hazards (Tentacruel, Cloyster) or an all-out sweeping threat (Keldeo, Cloyster, Sharpedo).
Overall, the most common type in high-level play is Steel
Hope this helped :)
