Which champ Should I buy?

Hey guys! So I just started playing ranked and I've come in contact with the so sweet, so loving League Community. SO LOVELY. And after playing 10 games I was placed in Silver 3. Now, if you were to check my match history, almost all of the games I've played, I've always been the support. Probably because no one else wants to be one. I just recently played my first Silver game and lost. Not very badly and I feel I did okay in that game. However, during the game I noticed that I should start playing other roles in ranked and I do not want to a one trick pony. Now just so you know I can play any role, obviously some better than others. For example I am a decent mid and a decent support but don't do so well when I am playing top. Bot and jungle is a hit and miss. If it is of any help, I already own these champions: Ahri, Akali, Ashe, Annie, Caitlyn, Garen, Katarina, Yi, MF, Nidalee, Nunu, Ryze, Shyvana, Sivir, Sona, Teemo, Tristana and Warwick.

Thanks guys!