Need Help Finding a Champion to One Trick

So I'm a fairly lackluster player. I find myself making so many mistakes, and Usually know when I make them. There are things I need to know about the game that I don't, and from what I've researched and learned, the best way possible to learn the game is by one tricking. This is because you are so familiar with your champion that you can solely worry about the game around you. I literally have stared at the champion list the last 2 days trying to decide who I would play. I have an account almost to 30. The mmr on this Gold account is trashed now from me deciding to play a different champion in a different lane 20 games in a row. I guess I'm scared to play a champion that doesn't have enough depth. Like I can't play annie more than 50 games or I would lose it. I also don't care what lane I'm in. I've played every lane, and almost every champ. I just notice a lot of champs stick out to me, then I notice a flaw or I give myself a reason to play some other champ. I think it would help if someone just told me that I could progress at the game with such and such champion. Anyways thank you!