That's a very open question... is your budget unlimited? Are you buying for fun or for profit?
You'd probably have to say tops is 03-04 Exquisite. Then I'd say 08-09 Exquisite. In no particular other order I'd enjoy opening 95-96 Finest, 96-97 Topps Chrome, 97-98 Metal Universe, 03-04 Ultimate, 07-08 Exquisite, 08-09 Skybox, 09-10 National Treasures (not out yet) and try and find a Tag in 09-10 SPGU. That's not 15, but it's a nice list