I think Rag, Cairne, and Sylvanas are pretty clearly the best legendaries in arena. After that, i think it depends on what kind of deck you're going for. I think pops values Deathwing more than me because he plays much more aggressively than I do in general. If you get into topdeck mode, Deathwing is a pretty good draw, but if you're still competing for card advantage, Ysera or Onyxia is usually going to be better. They're also pretty good topdecks, while Deathwing is horrible for card advantage, which I think makes him much worse on average.
I'd say my overall (tentative -- I have to think more about this) rankings are:
rag > cairne > sylvanas > ysera > onyxia > black knight > geddon > hogger > leeroy > deathwing
It depends greatly on the deck though, like I said. There are plenty of situations where I'd pick Alexstrasza, Thalnos, Harrison, Greenskin, or Illidan over Deathwing or Leeroy.