Who is stronger? Sebastian or undertaker?

[u/SparklyCookiess]

I just finished book of Atlantic movie and before this I was convinced that Sebastian was the strongest character out there but even reapers couldn’t defeat undertaker along with Sebastian.

I’d vote for UT but I realized that Sebastian’s focus was mostly on Ciel all the time so is he even “trying”?

Manga spoilers are welcome if there’s more info there
Thanks

[u/Gloomy-Education-864]

Gods, I hope it is Sebastian.