halls of stone (just brann really), occulus, azjol-nerub, old kingdom are the hardest to multibox
draktharon, gundrak are the easiest
rest are pretty much in between, once you do em once or twice without a single wipe they are all fairly easy. utgarde pinnacle for example wiped me at least 40+ times when I first started, but this week I have been running it daily (to get my DK a blue dragon lol) and haven't died once.

My go-to instance is violet hold, on days when I only have 30 mins to play I will usually run this instance in 20 mins or so.