Your motive behind the question is not entirely clear. Why do you want to know the highest/lowest skillcap class in order to pick a class. This is a game, pick the class that apeals most to you and you have the most fun with, full stop.
If you insist on asking the question I'll tell you a little secret. All 10 classes are more or less around the same skillcap, although somehow mages and rogues long argue that they possess the holy grail of skill cap (hey they are the most populus residents of durotar after all). Whatever what is not constant among classes and ofter changes the perception of people about the "skill" they take to play is:
a) Their relative strength. Often people would think that a weaker class takes more skill. Thats quite untrue, it is just weaker, and it means you will have less rating for the same amound of skill, but does not equal to a high skillcap.
Their learning curve. you dont pick up all classes at the same rate. Some classes may seem easy to get to a certain point, some are more liner while some others might be harder to get hold of them at the start. They all take a great effort to play at the very top level.
So if you are looking for a low skill cap class to pick up easier, then the correct question would be what is a class with an easy learning curve? I suppose thats not your point, or most likely you'd ask what is the most powerful class instead.
If you are looking for a high skillcap so you look "good and skilled" then sorry, there is no answer for you. All I can say is that playing the class you like the most and you feel more fluent with it is propably your best bet to establish yourself as a good player. Not everyone feels equal at home with different classes and having fun is a vital part in keeping playing and trying to improve.
In like 5 years playing wow I can guarantee you that I've seen a couple of absolutely amazing players from every class in the game. If you are an amazing top player, it will show no matter what you play
, even if you play a DK in season 5 you will stand out from the rest. Also I've seen a lot more wanna-bes playing classes that the general consensus at times believed to take skill to not make the slightest impact.
This comes from someone that enjoys playing diffrent classes and I have played most classes of the game at least on a decent level
Sorry for the wall of text, but thats first time I visit AJ after more than 6 months and got a bit carried away.TLDR version:
play the class you play and enjoy and not the one someone tells you to play on some internet forum. Thankfully all those 4 classes have distinctively different playstyles to make that choice for you easy.