Most champs have a counter. The term OP is relative in that case. As for the most powerful, most new champs are VERY powerful. Why? Because riot needs people to want them and needs people to buy them.
LeBlanc has some of the most ridiculous burst in the game, easily bursting players (who are several levels up) from full health to dead almost instantly, also her passive almost assures her escape when 1v1.
Vayne also is incredibly powerful. Her silver shots and passive superspeed allows her to focus on crit and attack speed rather than damage early on.
As for trynd, GP, Eve, and shaco, all of these players need to get reasonably fed/biult to be OP. The thing is, it is fairly easy to get fed as these players. As a matter of fact, you can look up on youtube a shaco trick that will get you first blood against the enemy jungler virtually 100% of the time. Kinda silly. IMO, there are some definite balance issues with many champs. (like shaco's box cooldown should scale differently so that he can't spam boxes right off the get go).
The thing is, most of the characters listed in this thread have an easy counter. Almost every last one of them can be countered by a good tank. However, due to recent gameplay changes by Riot, tanks are not neccesary to team comp and aren't even very viable compared to most bruisers. As a result.... balance issues.
As a general rule, new champs are OP by design to sell units. After the first patch or two, expect a nerf (like the Volibear nerf XD went from super OP to... "wow he sucks"). Some champs stand out above the rest, but if you play smart and your team doesn't feed them, they won't have much of a showing in the late game. Feed the, and you'll start getting the OMG OP!!! stuff.