I wanna play rammus but i dont know which skin to get, they all look pretty badass.

Check out the skins for yourself over at lolking and make your own decision.
http://www.lolking.net/models/?champion=33&skin=6

My own opinion is King Rammus. Failing that Full Metal or Ninja.

You're the one that's going to be looking at that skin every time you use it, you should be the one deciding which one you like the look of.

King Rammus, sadly, is no longer available. Failing that, I'd probably pick Chrome Rammus over Ninja Rammus (TMNT!) as a close second. The other three, IMO, are just boring; "Freljord Rammus" and "Molten Rammus" are the obligatory ice/fire skins that seemingly every champion has. (I know that's not the case, but they're WAY too common)

Full Metal Rammus, IMO, is one of the over-rated ones, as are most of the all-too-popular "robot" skins. I'd also avoid it for reason that the store's indicated it's been in the top 5 item sales ever since it came out like a month ago, so you can count on a lot of others having it; and it's not all that amazing to make up for it. (contrast to, say, Arcade Sona, which I'd use if I played Sona in spite of its popularity)

what about chrome rammus? ;(

Molten rammus ftw