Best is singular. But the one best piece that is available, rather than a group of pieces for the same price. Put all your eggs into the one best basket. The next consideration is the condition of the work of art itself, although, condition generally relates to the older art instead of contemporary work. Any alteration must have been performed in accordance with professional conservation practice and be documented by the conservator or the artist.
Authenticity is another problem that concerns older works of art rather than current pieces.
Despite the fact that the work was done by a living artist, however, the collector might wish to write a letter to the artist, requesting authentication and enclosing a professionally taken photograph of the work that the artist can keep. The letter should be addressed through the gallery to protect his privacy. These days, anyone can call himself an appraiser, from the framer or restorer who appraises high to justify his charges, to the antique dealer who knows a little about everything or to a potential buyer who appraises how to reduce the price. Picking an appraiser is like picking a painter or a conservator. Get a listed professional who is familiar with the type of art you have, and do tell him the purpose of the evaluation.