Great question! And great replies. I crafted my first legendary just yesterday, and it was Aeromancy. But before jumping to conclusions, here is how I would think about it.
1) First key question to OP is if it is a (relative) beginner's deck or not. Most legendaries require synergy from the deck, but some not.
2) Second key question is if you have place for a gold legendary or not. Maybe you have 4 great gold epics that you use.
3) Do you have a specific type of play in mind? Control, card advantage, whatever.
4) (you answered that) if you have a specific faction/leader in mind.
5) Any legendaries that you got from kegs.
Answers in my case:
1) - No, I was level 18 already, nerfed to 10, rose to 15. I had crafted a few epic cards like Ciri, Sigfrida, the dwarf-buffing dwarf, and more recently Aard. I play all factions and have a dozen of decks, so neither is particularly strong, but I am still unwilling to commit to one. So, the card to craft had to be versatile.
2) I had no easy place for more gold. It had to be silver. Also rules out some epics for me (the Mirror is very sweet...).
3) No, really a card which can decide the round all by itself.
4) No, but I give slight preference to Francesca and even with other decks I don't strive to draw the full deck (I play for fun not consistency), so e.g. Last Wish was not terribly interesting. Decoy is nice but I learned that I'll have it on level 20
soon enough in my case.
5) I really was lucky to get Igni in my very first keg.
Now, the conclusions:
* Geralt:Igni - the ultimate game-decider. Absolutely top, does not depend on anything else. You occasionally find it worth only 6 gold if the opp plays control, but against buffing combos it is unbeatable. Weather monsters tend to emphasize one row per round - with Igni weather decks will be your favorite opponents. The new D-shackles improve Igni even more (poor Hjalmar).
* Aeromancy - excellent synergy with Geralt:Aard (epic); also game-deciding. But that's 2 cards you need to have, and preferably a somewhat mature deck able to gain some card advantage (otherwise your weather will be just cleared by bronze Clear Skies).
* Last Wish - I don't have it, but I feel really need to have something valuable that you want to get from your deck.
* Ocvist - too often I just kill him at once when I see him on the field.
* Yennifer - seems very powerful and versatile. Ideal for board-spamming decks or against them, but on average may not be the game-decider you are looking for.
Hope that helps.