At the risk of sounding self promoting my favorite building is the one I work in.
It is a 500 year old medieval manor house called Gainsborough Old Hall. It blows my mind that such an important building is almost unheard of, yet so many great people have lived/stayed there and parts of it that have been virtually untouched for 400 years! I'm far too close to the subject matter, but for historical signifigance it has been visited by Richard III, Henry VIII (owners liked to keep all their options open
) Katherine Howard and Katherine Parr (who was first married to one of the owers sons), John Wesley preached in the hall several times and the pilgrim fathers worshiped there!
Ok, I'll come down off my soap box now
but just for the record, I can't recommend it enough!