Disregarding the obvious trolls where people request stuff like low-levelled legendaries and such, the most sought-after pokemon, besides version exclusive legendaries and event pokemon, would probably be certain dream-world pokes like vulpix, poliwag, bulbasaur, eevee, torchic, dratini, and since pokes like hydration vaporeon, magic bounce espeon, speed-boost blaziken, and multiscale dragonite are in high demand not to mention the the always common drought-tales and drizzle-toed...some other dream-world ones like shroomish and bagon are pretty popular as well...If you have any of those, you should be able to get some pretty good stuff in return...but of course if you have shinies of any rare or strong pokemon like any starter, zorua, any powerful dragon poke, beldum, save any shiny legendary, the people will completely eat that stuff up...If you really have any of those pokemon, you should be good...