It's so hard to have an absolute favourite that I may have to slightly dodge the question, but Durham Cathedral has to be up there, as does my current place of employment (when not furloughed!), Salisbury Cathedral.
Years ago, when I worked in local government in North Yorkshire, I was chatting to one of the staff of the Yorkshire Dales National Park Authority at the end of a meeting. He said he had worked in Canada for a while and really got into ice hockey. He said he had no idea that Durham then had, in the Wasps, one of Britain's best teams. I said that I would take him to a game there one Sunday, and we would go early enough to visit the cathedral, to which, it had transpired, he had never been.
When we went through the west end entrance to the cathedral, he was overwhelmed with awe by the sheer massiveness of the Norman architecture as he gazed into the distance, as it were. he said, "It just makes you want to say, 'Jesus Christ!' " My predictable reply was, "Well, yes, that's sort of the idea!"
He enjoyed the Wasps' hockey match too!
For secular buildings, I agree that the Piece Hall takes some beating. In my local government days, I had to attend a few meetings at Halifax town hall, which prompted me to say to a female colleague that it was a shame that she could not, at first hand, appreciate one of the finest expressions of Victorian civic pride that I had ever had the privilege to see. I was, of course, referring to the quite magnificent gents urinals.