saw an interesting chart over on reddit
This pic shows the win % odds you have with all of the characters. Note that a rengar has 9.38% lower odds of winning a game at this point... with diana not much better. The chart also solidifies that mf is the current king (queen) of adcs.
One stat I found really interesting that if you go are garen, you are 7.69% more likely to win, which sounds a little crazy. Keep in mind that these numbers don't have a 1 to 1 correlation to games played. For instance, karma is played very seldomly, but factors in at -7.89% to win due to her overall win rate when she's played. When looking at data like this, its best to consider what champs are most commonly played. If those champs have a higher win rate %, then that's a pretty good indicator that they are possibily overpowered in the current meta.
Amumu, for instance, is +5.74% more likely to win a game and is utilized quite a bit when not banned in mid elo games. The reason being, ofc, that his initiates are so strong and require a relatively low skill cap, both for amumu AND for the rest of the team (eg oh shit amumu is going in, FOLLOW HIM!). So, its generally a good idea to pick up amumu when possible if he's free unless you have another great initiator. And ofc, provided you or someone on your team is a competent jungler.
By the same sort of logic, mf is a very intelligent pick in the current meta now - combine her ult with an amumu jungler and well - boom. Anyway, the pic is worth taking a look at and thinking through a wee bit.
Other thoughts - I think zed is a wee bit overpowered. Now, I'm not exactly a good zed - kinda far from it, but playing 1 game last night I was in a position where there was a 100% chance I could pick off either the ap carry or the ad carry every single time. I really don't think there should be characters with a 100% chance to take out a major damage dealer. He has an extreme gap closer with flash/ult/shadow - so its pretty damn hard to stop him from killing a given target. And his burst is just sick and something that certainly can't be itemized for on adc. There are better picks for mids, ofc, but I'm pretty confident that having 100% chance to take out ad/ap carry is OP. And that's what zed brings even in an unfed form. That said - per the current stats - you are 3.16% less likely to win if you pick zed. I think the solution is a nerf on his ult, but with him currently under performing stat wise, I don't know that we'll see that.