Actually, there are only four sites, the Dolmen de Menga and Dolmen the Viera are next to each other. I would recommend this site if you have only little time, it is only about 2 km from the train station and the city centre. One hour is enough to visit the two dolmen, the site is accessible from 9 am to 6 pm without siesta break (Mondays closed).
The Dolmen el Romeral is another 4 km from the city centre, but I have no information on public transport, we travelled by rental car.
If you have more time, El Torcal might be interesting. It is about 10 km from Antequera to the visitor centre, a karst landscape, an one hour trail would be enough to get an impression.
The second natural site is not accessible, but you have a perfect view to the "lying face" from the Menga Dolmen.