P668 is a good starting point.
"Best" really depends on the type of plant and your personal preference. Kind of like asking what's the best recipie for cookies or a martini - you'll get a bunch of different answers.
Order from the Orchid Seedbank Project. They sell a good variety and don't carry a bunch of stuff that you don't need. Most they carry already have the agar included and are in one liter packages for about 6 bucks. Other lab supply companies will gouge you for shipping.
Choosing one is tough, and confusing. That's why you should get two. Look on the OSP site in the technical data tab for a blurb on selecting media for defferent genera. If you get two very different formulas, almost anything you sow will come up on at least one of them.