rogue>feral>deathknight>ret>enchanement>warrior is skillcap order of melee from highest to lowest.
If we compare the amount of deathknights in the top 400 teams to warriors we see a huge difference, only an idiot would say deathknight are poorly represented because they are weak, and as there is no shortage of deathknight comps, It leaves only the possibility that most deathknights simply cannot play their class up to par. As there is no difference between the players of each class these days with all the alts etc, We can infer that this means deathknight is harder to play well above 2.2k than warrior.
Well my list with hardest to the easiest would go like this
anyrogue>enchanement>feral>uhdk>>ret>warrior>frostdk is skillcap order of melee from highest to lowest.
and I think the most skillcapped spells for melee classes are
Darksim > > Capacitator Totemw/oglyph >> Smoke Bomb >> MSR >> Grounding Totem >> Shockwave
But those are just opinions ^^