Pantheon

I'm no architect nor historian, but I have always thought that the Pantheon, the temple to all the gods, is the greatest and oldest of all extant buildings, the most ingenious of the classical era, begun by Augustus and completed by Hadrian, who, some say, designed the great dome himself; the grandeur of Rome all in one place. No photo can capture it. But it is worth many.

From the back side it's not all that impressive--ignoring the fact it's 2,000 years old

The facade, begun by Augustus and his chief architect and son-in-law, Marcus 
Agrippa, is more promising

It's when you stand at the great 20 foot original bronze doors and gaze up at the 
sunlight blazing through the oculum, 140 feet overhead, and understand that the 
width of the rotunda is exactly equal to its height...that it begins to overwhelm you

As I said, no photo can do it justice

1500 years after the Pantheon was built, Brunelleschi--
who was attempting the great dome in Florence--was 
permitted to drill into the Pantheon's dome to see how 
it was done; here, I think, are the holes he left 

Rafael's tomb

Another exterior view