As amanico said, Via Condotti is the place to be, you can find Patek, Rolex, Cartier, Omega and Tudor boutiques. Piazza di Spagna and Via del Babuino have other boutiques and multi brand ADs, there you can find JLC, Hublot, Zenith, Panerai, GO, UN, Breitling, U Boat, VC and more
The AP boutique is also not that far from there, I don't remember the exact name of the square but it's near Via del Corso