I'm still debating what legendaries to craft. I picked up the twin taunt (if C'thun has 10+ attack, summon a duplicate copy onto the board guy). I have like 9k dust or so still. Thinking about nice Rag, and the 1-2 mana one that changes what it is every turn until you play it (might change every turn, as I have only seen it once and I killed it with a BGH).
N'Zoth if you don't have it is clearly going to be used in a lot of decks. Most decks have enough deathrattles to justify his use in almost any deck, whether it's built around N'Zoth or not, and I think when the meta settles, it'll be in quite a few decks.
Malkorok is obvious if you like warrior, I think. I crafted him and he is excellent in any version of a Tempo based warrior (like dragon warrior). Not so great in control warrior, but I have so many control warrior games behind me, now that tempo warrior builds are pretty viable, that's all the warrior I've been playing (and it has performed quite well on ladder for me, as you are often enough ahead of C'Thun decks that they are on their heels all game plus warrior has really good tools against the aggro decks)
Light Rag, I don't like so much. That heal / N'Zoth deck has some matchups that are as bad as freeze mage vs. warrior. But if that looks like a deck you want to play, go for it.
Shifter Zerus is a fun card, doubt it will end up particularly competitive, but this and Yogg are both in that fun to play if you have spare dust category.
Fandral is going to end up in pretty much every druid deck just because even if he only hits on one card it's a pretty big deal. I got him as one of the 4 I opened, and he can win games if the opponent doesn't remove him.
Xaril is the same for rogue decks. I crafted this, because the Old Gods Rogue playstyle is one I like better than previous rogues, plus the meta has slowed just a little so Rogue can actually survive the early game. I'll be surprised if there is any competitive rogue deck without Xaril. I had a grand total of 17 ranked wins with Rogue prior to Old Gods. It's fun to be able to experiment with rogue. The 'old rogue' felt like you needed to spend a week solid playing it just to get to a point where you were making the right decision most of the time.
Similarly, if you don't have the deadmines dude... I forget his name. Defias leader dude. He is also going to be in many rogue decks and since he's in the Classic set, he will be around forever. A good craft now if you have any rogue preference. (remembered... VanCleef).
Hogger is "C'Thun Protection" and a definite consideration
Soggoth is a card that's on the edge for me. I can't help but have this feeling in the back of my head that there will end up a deck he ends up playing a really big role in. But that deck has not been discovered yet. Perhaps something to experiment with if deck building and experimentation is your thing.
I'm not sure any of the others are worth considering.The mage and warlock legendary seem universally considered unusable. I have seen the Shaman legendary in some fatigue / extreme controlly shaman decks, but I can't imagine these stay viable forever as decks get refined. Cairne seems good now, but I bet he will filter out eventually, but he's a potential strong craft too.
That's my 2 cents.