It depends on the venue itself as well.

The loudest band I've seen live was Full of Hell in a 300 capacity venue. The second loudest was cradle of filth at Ozzfest 2005.

Although you can technically measure the decibel levels, I think the "loudness" of a band is subjective.

That being said, Sunn O))) at the caverns in TN must have been a mind fuck.