Baalbek Fantastic preservation, incredible architecture and the influence on architecture through the ages is extremely important - highly influential on Renaissance and neo-classical artists. One of the first sets of drawings to reach the North was of Baalbek and was a spark to neoclassicism.
Limes Hadrian's Wall - mainly.
Pompeii/Herculaneum This one is perhaps the most important. The art-history/archaeological importance of this can not be overstated. Furthermore it gives a look into everyday Roman life.
Pont du Gard Iconic and monumental. Evokes the unrivaled glory of Roman civic works.
Leptis Magna IMO, the best Roman site in Africa.
Villa Romana del Casale Mosaics.
Villa Adriana, Tivoli Again, the importance of this site can't be stressed enough in the development of the countryside villa as we know it and the later developments of that. As a site to visit, it is extremely interesting.
Are we voting for Rome here? If so, add that as a YES too.
I wish Maison Caree in Nimes was listed already, because that would be a solid YES.
El Jem - I think ampitheatres are covered by other sites, but I do find this one really impressive. Rome has the Colosseum of course. Do we need another? (Arles, El Jem, Verona, Tarragona, Merida, Orange, etc.)
P.S. Why isn't Italy fixing up its Ostia nomination? That is such a crucial site and I think the WH List is all the poorer for it not being included. Other than Scrovegni Chapel, Italian sites lately have not been up to the level of a site like Ostia.