I remember the old times.
Nozdormu is a joke card. Don't ever play it, it sucks.
Alextraza is really good in some decks (it actually gets played in constructed from time to time! Wow!). Control warrior comes to mind. I'd mention handlock, but I'm not sure whether or not the demon variety ever uses it, and that's the meta handlock currently.
There was a saying that playing Ysera on an empty board would win you the game. I think that's mostly still true. In a slow, control-ish metagame, people would absolutely play it. They don't, though.
Malygos is really, really scary. Constructed is so finely-tuned that if someone plays malygos against you and you don't remove it or kill him, you absolutely will lose the game the next turn... Though controls warriors with 20+ armor can generally stay alive.
Onyxia is good, but it sucks against any type of board clear. For that much mana, I'd rather play something else. Still good, though.
Deathwing- Nope. I'd rather play twisting nether. And I'd almost never play twisting nether. It can only work in a slow metagame, but in a slow metagame, you can't afford to enter top-deck mode. Card sucks.
For the new ones-
Nefarian- Hey, look. It's 9 mana 8/8 with bad thoughtsteal! Demon handlock cuts 4 mana 8/8 because it's not good enough... This card is hopeless. I'd rather play Ysera. I guess that in a super-slow meta, you could try to play both.
Chromaggus- If it stays on the board long enough it's pretty good? I think I'd rather play Kel-thuzad most of the time, but I bet that in a slow enough meta it can work. Value.
I'd probably pick Onyxia or Ysera in arena.