The central temples of Angkor are ostensibly split up into two routes—not that you are bound to tour them as such—consisting of the Small Circuit and the Grand Circuit.
The Grand Circuit is in fact just an extension of the shorter Small Circuit loop, taking in a few more sites. Both are on good, sealed roads which collectively knock off most of the key temples of interest within the vast Angkor Archaeological Park, excluding remote places like Banteay Srei, Beng Mealea and Kulen Mountain.
While there are more temples than you’ll ever manage to squeeze in to your first visit to Angkor, picking a few from the collection of temples that sit on the Grand Circuit fits very well into a half-day tour and should be high on your list of places to visit after the big three (Angkor Wat, Ta Prohm and the Bayon).
The main temples of the Grand Circuit are: Preah Khan, Neak Pean, Ta Som, East Mebon and Pre Rup. This handful should help form the focus of your Grand Circuit tour, being the most interesting historically and visually with a variety of architecture.