Traveling in Europe you see a lot of cathedrals. I still go into most of them but sometimes I feel like if you've seen one you've seen them all. Having said that, I've come up with a few awards for the ones I've visited so far.
Most famous: Notre Dame, Paris
The Biggest/Most impressive/My Favorite: St. Paul's Cathedral, London (tied for best view)
Honorable Mention: Karlskirche, Vienna
I wanted to make an award for the most steps but I'm not actually sure who would win that one. Probably St. Paul's or the Sacre Coeur - Karlskirche had an elevator!