Pantheon

Originally commissioned by Marcus Agrippa under Augustus and rebuilt in its current form by Hadrian around AD 126, the Pantheon has survived nearly two millennia largely intact thanks to its continuous use, first as a Roman temple to all the gods and later, from the 7th century, as a Christian church. Its dome remains the largest unreinforced concrete dome ever built, with a central oculus open to the sky that floods the interior with light and occasional rain, and reviewers with a 4.8 rating from over 285,000 reviews consistently call the sheer scale of the columned portico and dome unforgettable in person.

Free to enter but increasingly requiring booked timed slots, it's a quick but essential stop that rewards visiting more than once at different times of day for how the light changes inside.