You need to find the champion that resonates best with you, then Main that, rather than trying to force a Main; I started the game with Jinx on her release, and whilst I'm not bad at her, my first Main was Leona, because I play fairly aggressively and am not bad at making engage plays (i.e. when to engage vs back off, who to engage on). In the games you win and do well in, look at what you did that enabled you to carry (did you land some really clutch skillshots, or repeatedly single out and kill the enemy carry), then find the champions that are good at that.
Simple champions can get you a fair ways in lower Elos, but they tend to drop off fast as you climb, because simple to play usually means simple to counter.
- Annie has a nice simple rotation that is easy to land if you can get in range; any poke mage with a hard cc can remove her with 0 risk.
- Warwick has a really healthy clear early for a jungler (seriously, I'd recommend his as everyone's first jungler right now), but hard CC shuts him down and he can't do much to tanks on his own. Also, his clear, whilst healthy, is slow, and the enemy jungler can easily take camps from Warwick's other quadrant, denying him gold and exp.
At the end of the day, the goal of Ranked is to win and climb (keep the 'purely for fun' shit in Normals); with that in mind, play with what wins you the most games.