Uhh good question. I think it depends? Pokémon like Celesteela are absolutely enormous (watch its anime episode if you have not already) and seem like they'd do devastating damage even to legendaries. Guzzlord can also swallow up mountains and buildings, so its threat level from that alone is higher than many legendaries.
Overall, though, I think legendaries inch a bit further. Lots more variety and possibilities for destruction amongst the legends we have.
....& I like UBs because it has Poipole, but legends for Diancie and Ho-Oh :>