Sunwell by far, pre nerf's it was hard as shit, it's a beautiful zone and has beautiful music, still is, I was also at the top of my game and height of experience in SW, I still solo it weekly just because I love and miss the place (alongside solo'ing TK/SSC/BT and so on also) I'm incredibly proud of my M'uru pre-nerf kill and KJ.
I wasn't raiding properly during ICC or DS so I can't really say much about them apart from casual stuff in ICC, but I still prefer Sunwell by just an edge.
SoO is not bad, wing 3 is pretty cool, only doing LFR currently so can't say much about wing 4, should really start doing flex...TOO MANY ALTS!