Still the largest un-reinforced concrete dome on Earth, the 2,000 year Pantheon was built to symbolize human faith in all Roman gods. A display of architectural genius, the front is a many-pillared portico, while the majority of the building is the aforementioned dome, a design that has been copied extensively through Western civilization. Although quite a sight from the outside, the interior, particularly the evenly spaced ceiling coffers and the tiny oculus (a hole in the roof) that still lights the space, is quite an achievement for the time.
Converted many generations ago into a Christian church, tourists keep this truly essential landmark abuzz daily. Touristy or no, it's your duty in a society that owes many of its developments to the Roman empire to see it at least once.