Going in a less streamlined direction than @Caperfin & excluding the question of role, since it was only asked for class, I would go with Druids.
Not because I play one & not because I think I a have mastered it (which I don't), but - for example 1 - because in Solo / PvP Situations you are at your most effective when utilizing all three of your specializations.
A cat is not a rogue, a bear not a warrior, a boomkin not a mage & Resto not a Priest. But juggling Forms according to your needs, controlling your mana well, crowd controlling, healing yourself, throwing a harmful spell or two makes for an efficient & unique but difficult to master Playstyle in those situations.
This perspective, of course, disregards group activities like dungeons and raids, that depend on you filling your role, but I would argue that even here, the difference between a masterful druid and a "meets expecations" druid can make all the difference in tricky situations.
I'll paint a picture for example 2:
You are at Major Domo, you are MT'ing as a bear. Mr. Domo decides to port you. While jumping off the lava pit, you take the seconds you gain without being physically hit to shift out of Bear form, use Nature's Swiftness, Combat Res a healer, Innervate the healer you ressed, go back into bear form, taunt and continue tanking.
I interpret a high skill cap as "getting the most out of your class". To pull this move off, the Druid needs great awareness, quick reflexes, precision targeting, good positioning of the boss, precise character movement and s specific spec.
Nobody would be angry if he just picked up Mr. Domo again and continued tanking, that would have just met the expectation of the role, not the capabilities of the class.
Rinku < the Druid > - Gehennas, EU