I play all of them and i want a skin so what should i get ? I mean i want to chose betwen Hyena and Firefang 
because i like both of them . And get the Shockblade 
with one of the skins . Or should i get only Demonblade 
? I have 2168 RP .
Unless you play tryn a lot I wouldnt get demonblade. Its a decent skin, but tryn is rarely ever a strong pick into any comp. Zed and WW are both pretty good and you'll have many more opportunities to play them!
I personally go for flare (dohoho) on my skins so Firefang>Hyena.
Despite that, i'd say go Demonblade Tryndamere, one of the best legendaries in the game (even after it's release a long time ago) and chances are if Trynda gets a TU or a VU it'll look even more badass.
You could also use this to help you decide: YoutubePreview/ModelViewer DBTrynd: https://www.youtube.com/watch?v=lEWjnWJp9LU / http://www.lolking.net/models/?champion=23&skin=4 FFWar: https://www.youtube.com/watch?v=O6FBIiiHso8 / http://www.lolking.net/models/?champion=19&skin=6 SBZed: https://www.youtube.com/watch?v=aLGc5pZkl28 / http://www.lolking.net/models/?champion=238&skin=1
