What is the earlist major islamic building that still exists today?
The Dome of the Rock is the earliest major Islamic building that still exists today.
The Dome of the Rock is an Islamic shrine and major landmark located on the Temple Mount in Jerusalem. It was completed in 691, making it the oldest existing Islamic building in the world. The site's significance stems from the religious beliefs regarding the rock, known as the Foundation Stone, at its heart. See related link for more details.
Hadrian. He really built the pantheon onto a small temple of Marcus Agrippa which presently exists as the portico of the building. Hadrian was very modest and didn't bother to put his name on the domed building, but left the original title on the portico which apparently applies to the major structure 'Marcus Agrippa fecit'.