The best light for hydroponic growing, like that of many other methods of growing, would be an H.I.D., or to be more specific, a HPS or MH. You could even throw in a Mecury Vapor lamp, because it produces an abundance of ultra violet light (even more than that of an MH) which will cause the plant to produce more THC resin to protect the seeds that are to supposely grow, (you are aiming for a sinsemilla effect, meaning no seeded bud) from the sun's UV rays (UV-A, UV-C, and UV-C).