In our lab., Hyponex medium (2.2 g/L) supplemented with 1 g/L activated charcoal has been used for more than 15 orchid species (seeds, rhizomes, PLBs).
This depends strongly on the species. Knudson C is a well established first guess, but there are several orchids not performing very well with it. Could you give species or genus names you are especially interested in? Conservation, by the means for a collection or do you plan to re-introduce the plants?