Comment below rating threshold, click here to show it.
Now that Diana has been nerfed, noone is really OP so to say, any champ can be "OP" if the summoner is good. Example: I have spent countless hours perfecting my Lee Sin game, in my hands, he is ridiculous. With a couple wards and my wriggles I can 1v3 with ease in the jungle, if I am being chased good luck pinning me down and because of that, I have had people cry OP on Lee Sin. Another great example of this is Veigar, a summoner that is proficient at last hitting can farm that Q up and damn near one-shot any squishy AD carry, so in the proper hands Veigar could be considered "OP".
If you are looking for champs that were broken like Diana was, you are out of luck, Riot is pretty good at catching ridiculousness like that. The closest thing we have now would probably be Darius and that is solely because his ult is broken, but on the flip side of that it doesn't do a **** TON of damage, it's just a good finisher, it's like with a Darius on the opposing team you all lose a little bit of total HP because their team only has to kill you most of the way and let Darius ult you to death.
Another champion that I suppose could be considered "OP" would be Blitzcrank, but crying OP is a bit extreme in my opinion. The only two things that make Blitzcrank strong are his Q and his E, his Q is that nasty go-go-gadget grab he has that seems to be able to reach half way across the map, and his E is only powerful when used in conjunction with the grab because it will pop you up and let the ADC snag an easy kill.
Malphite is another champ that a lot of people cry OP on, and it is for the reason that he is just damn hard to kill. Now I personally don't mind Malphite much, his early harass can hurt a bit, but his damage falls off quick, especially with a little MR built. Malphite is just annoying, his ult is a pain in the ass during a team fight, and the fact he is damn near impossible to kill when built full tank makes him a champion that is banned in ranked play a lot, not because he is OP, but because he is tiresome to deal with.
There are numerous easy-mode champs that some people cry OP on, but it's only because they don't understand the champ and how to counter them. Some of these are Xin'Zhao, Riven and Akali, they are all strong easy-to-play champs, but with some understanding and some strategy they can all be countered. So in summation, there really aren't any straight broken champs like Diana was (and Zyra before her), you just need to find a champ you are good with and make them OP in your hands.