Raiding 10man,

Halfus - 3/10
VNT - 6/10
AC - 9/10
Chogall - 8.5/10

Magmaw - 5/10
Ommo - 7.5/10
Chimearon - 4/10
Atramedes - 5/10
Maloriak - 3.5/10
Nefarian - 9/10

Conclave - 5.5/10
Alakir - 8/10

This is mostly coming from a healer POV, im sure DPS or tanks won't agree on some of them ;>

Sinestra - 10/10

And as for sinestra, it might not be the hardest fight in terms of shear amount of mechanics; but they are definitely the most unforgiving. And let's not forget the pretty solid personal DPS/healing requirements.