I think for Bvlgari there is not one specific piece, but rather a distinctive style (perhaps with the exception of the b.zero1 ring which is only 10 or so years old). Their distinctive style encompasses tubogas jewels, coin jewelry, the BVLGARI logo, and the parentesi motif. They also popularized the use of non-precious with precious materials (steel with gold, ceramic and gold, rubber with gold, etc).
If I wanted one iconic piece it would be a bezel set ancient coin necklace in yellow gold with curb chain links(either a sautoir with multiple coins at different stations or just one coin).