The Duomo
The great cathedral of Milan, aka the Duomo, was begun in 1386 and, as everyone insists on telling you, took more than five centuries to complete. They should have stopped sooner. Much, much sooner.
The Duomo is indeed an impressive structure; "awe-inspiring" might not be too strong. The sheer physical size is staggering, let alone the quantity and quality of skilled labor that it represents.
It is not, however, beautiful. Mark Twain thought it was, and went on about it at great and admiring length, but then Mark Twain thought Titian's Venus was obscene.

The basic layout and proportions aren't too bad, though rather on the blocky side, lacking the soaring lightness of a Gothic cathedral. But, quite simply, they stuck too much crap onto it. The cathedral at Amiens is beautiful; so, in a different and simpler way, is the oldest of all the great Christian churches, Haghia Sophia. So is Notre Dame, at a lesser level. The people who built those churches knew when to quit. The people who built the Duomo didn't.

And the closer you get, the tackier it looks. Apparently the idea was to cover every possible square foot with ornamentation - especially statues. The books say there are over 3000 statues on the exterior alone, and I believe it. They aren't incorporated gracefully into the overall design, either, like those of the great French cathedrals; they just sort of hang out there. And where they didn't put statues, they filled in the space with all sorts of elaborate and generally tasteless gingerbread.