Hearthstone's legendary cards are always a risky investment. Crafting one costs an incredible supply of arcane dust, so much that you'll need to disenchant four normal legendaries to craft a single one of your choosing. Assuming you don't have a golden version, you'll need 1600 dust which is a lot unless you're sacrificing other legendary minions to get there. Each will net you 400 dust, but if you'd rather hang onto those, you can always turn to epic cards which net you the second most dust per card. We've already prepared a list of epic cards suitable for those needs.
We've done our best to properly sort each card, but do keep in mind that the meta could shift and cards can suddenly change in usefulness. And that's before you even consider the fact that new sets could potentially increase or decrease the power level of each of these cards.
We highly doubt that any of these cards would see nerfs come the next Standard rotation, but do keep in mind that Blizzard will almost assuredly make changes to Basic/Classic cards, so you might want to save those up for maximum value.
It's also worth noting that a collector should probably disenchant legendaries at all. They're incredibly hard to obtain with only one guaranteed in every forty packs. Therefore, you're far more likely to craft the ones you want rather than simply opening them all via packs. So make sure you really don't want a card before you break it down into dust.
Cards to Keep
Under no circumstances should you disenchant these cards. They're either already seeing a ton of play or likely will in the near future.
Aya Blackpaw - Even somewhat light Jade Golem decks like to run Aya and that probably won't be changing.
Raza the Chained - Reno Priest isn't as good as many thought it would be, but Raza should be an important part of Control Priest going forward.
Inkmaster Solia - Reno Mage is arguably the best Reno deck at the moment and Inkmaster certainly helps its case.
Cards to Consider
These are borderline cards that are either seeing limited play at the moment but could be important if the meta shifts or they're buffed by subsequently released cards or the upcoming Standard rotation.
Shaku, the Collector - Ran in some Miracle Rogue decklists, but certainly not required.
Wickerflame Burnbristle - Doesn't really see play right now, but if Paladin was in a better place overall it very well might.
Finja, the Flying Star - Sees some play in current Anyfin Paladin decks.
White Eyes - Very good in Control Shaman. Might not be the best right now, but if it does catch on, this will certainly be involved.
Cards to Dust
All of these cards have a gimmick at their core or just aren't competitive enough to see play unless they get a significant amount of support in the next set.
Hobart Grapplehammer - Too slow for the current iteration of Warrior. Maybe one day weapon Warrior will be a thing, but that doesn't seem to be the case now.
Auctionmaster Beardo - Not enough value.
Genzo, the Shark - Too weak considering it prospers in an aggressive deck.
Knuckles - Seems cool, but with the lacking Grimy Goons mechanic this just isn't good enough.
Madam Goya - Too much RNG for constructed play.
Wrathion - Dragon decks are about to get a lot worse once Blackrock Mountain rotates out. If Wrathion doesn't make the cut now, he certainly won't then.
Don Han'Cho - Easily the closest legendary to being in the Cards to Consider category. It's not that this is inherently bad, it's just really slow.
Krul the Unshackled - Not enough great demons at the moment and doesn't work well unless you're absolutely sure your opponent has no board clears.
Mayor Noggenfogger - Gimmick.