It really doesn't matter which class is the most complex in terms of mechanics. What's most important is what best fits your play style/mindset. If the most complex class in the game just so happens to suit your play style best, then I would actually consider that your "easiest" class. Playing a class that doesn't suit you would be your "hardest" class to play.
If I had to guess which specs have the highest skill cap though, I would say it's those which have to worry about multiple things all at once. So not a healer that just focuses on heals, not a DPS that just focuses on damage, and not a tank that just focuses on tanking. Highest skill cap would be an offensive tank that has to not only swap guard at breakneck speeds to save lives, but also contribute meaningful damage bursts at the crucial moments.
Of course, an offensive tank might actually be more viable and thus "easier" than a gimp spec so... there really is no clear answer as to which spec is hardest/highest skill cap.