Underworld was a fine story -- I'd never call it dreadful. Indeed, there's only one story that I'd call dreadful. It's still in the list below, although I've saved The Armageddon Factor, another fine story.
The list is now:
The Power of Kroll
The Creature From the Pit
The Horns of Nimon
I think we can all see where this is going. I've heard people defend nearly every TB story in my time except for one. We all know the one I'm talking about. No need to mention names.
As an aside, both The Power of Kroll and The Horns of Nimon had redeeming qualities. I liked, at least, how HON began. POK is always a funny one for me because DW used to air at 11:00 p.m. on Sunday nights here in Chicago. My brother was always making fun of me for watching it. He happened to walk in while the Swampies were doing their ritual dance and chanting "Kroll." My brother just looked at me and said "Dude, really? Even you have to admit..." etc, etc.