The simple answer: Whatever your vendor or source recommends and stocks is best. Orchids, in general, will grow satisfactorily in many different potting mixes if watering and fertilizing are adjusted appropriately. That is, if the basic requirements for moisture, root aeration and support are accommodated, the most readily available media in your particular area is probably the one that has proven to work the best. Orchids are grown today commercially in a variety of media, from straight fir bark, to sphagnum moss, to the increasingly popular peat-based mixes best exemplified by Pro-mix. Watering frequency is generally inversely proportional to the porosity of the media used; in other words, the faster the mix drains, the more often you’ll have to water. Complicating the answer is the knowledge that many, if not all, of the most often seen potted flowering orchid plants in garden centers and other sales venues have been potted into larger containers in fresh media almost immediately prior to shipping. This is for very practical reasons: the container and fresh mix look more attractive to consumers, and the plant can be grown in the smallest possible pot until the last minute, keeping bench space fully utilized. Such plants need to be carefully watered with the increased water needs of the flower spike balanced against the lower potential uptake of the disturbed root system.