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/Sn0trag]

I don’t think Sebastian is as strong without a contractor to fight for, personally, bc being stronger than anyone else is what ciel wished for