Pantheon. It was actually the second time I have explored the inside of the Pantheon since I have been in Rome, but I love it. It is so huge that it is overwhelming to take it all in.
"M. AGRIPPA.L.F.COSTERTIUM.FECIT” which means “Marcus Agrippa son of Lucius, having been consul three times made it”. How cool is that to have your name inscribed on a building that has lasted for centuries! Emperor Hadrian built this Pantheon to replace Marcus Agrippa's Pantheon that burned to the ground.
*Added information from "Rome.info Roman Pantheon." Rome.info Rome tourist information, Ancient Rome travel guide. 19 May 2009 http://www.rome.info/pantheon/.