Sacado de reddit:
Hello folks of /r/hearthstone, recently I've been noticing a lot of "Should I DE this legendary or should I keep it" threads, I'll provide my input and opinion on each legendary, which uses I know it has in the current meta and my recommendation to keep or DisEnchant depending on if you need it or not. (WARNING: long ass list)
Disclaimer: this guide is mostly aimed at people with a semi-tight budget that care more about being able to play the decks they are interested in playing atm rather than completing a collection, if you don't agree with something I listed, please provide feedback in the comments to explain what you didn't agree with, downvoting without feedback won't allow me to improve upon the list 
Tip: Keep in mind to only actually disenchant cards when you need the dust, cards can become more useful over time so always keep that in mind, just make a list in your head of which cards you would first get rid of when you need the dust.
Cenarius - Druid
Used in many iterations of Druid decks, mostly ramp and a solid legendary overall, does great work topping off your curve and can be great for playing turn 9 on an empty board, especially now that hunters have gotten a decrease in popularity.
Recommendation: Keeper if you'd at any point like to play druid
King Krush - Hunter
Currently not used in any meta-decks, has some place in very niche self-made aggro hunters but usually doesn't make the cut.
Recommendation: Disenchant ASAP unless you are currently using him in your deck
Archmage Antonidas - Mage
Has a place in many mage decks, from aggro to control, to trump-style variants to anything really, very solid card overall that combo's well with a mage's low cost spells.
Recommendation: Keeper if you even considered playing mage at all.
Tirion Fordring - Paladin
Has great value in any paladin deck at all, consistently in over 90% of Paladin deck-lists I have seen, Keep it even if it is to keep the possibility of trying it out open, it's a great value card.
Recommendation: Finders keepers.
Prophet Velen - Priest
Not usually found in priest decks but could definitely come back at some point, doesn't usually live until the next turn though.
Recommendation: if you're a priest Fnatic, keep, otherwise DE
Edwin VanCleef - Rogue
Has a place in many tempo / miracle / malygod decks alike, coining him out is still as amazing as ever.
Recommendation: if you're a rogue player, consider keeping, he's not required but could be of use
Al'Akir - Shaman
Found in many Shaman decks these days for his amazing burst potential with that +6 damage from rockbiter, great for killing combo's and decent in constructed overall
Recommendation: keep if interested in playing shaman
Lord Jaraxxus - JARAXXUS
You don't fuck with Lord Jaraxxus eredar lord of the burning legion, however he is a tad slow for handlock these days and he has no real place in zoo.
Recommendation: If you're a Warlock- eh, Jaraxxus fnatic, keep it, if you just play Zoo or think he won't fit in your deck, DE if you really need the dust (but beware of your crimes)
Grommash Hellscream - Warrior
Staple of Warrior control and found in some aggro Warrior decks
Recommendation: Simply put, you NEED this if you plan to play some serious warrior, only DE if you plan to not touch the class in the following 2 years.
Neutral Legendaries:
Alexstraza
Used in many a control deck across many classes, a staple for warrior control, handlock and old-fashion Freeze-mage.
Recommendation: Keep if you ever played a control deck in your life
Baron Geddon
Good vs the zoo meta in warrior control and Amaz-style priest
Recommendation: If you enjoy countering meta's this card is great vs aggro, he will likely keep popping up in places, I recommend keeping.
Bloodmage Thalnos
found in over 80% of control decks to date
Recommendation: Keep, no questions asked.
Cairn Bloodhoof
Used often in control but has been falling off lately because it can be rather slow vs some match-up's and countered heavily by shaman, which is on the rise. That said he is still a solid staple of many control decks, I crafted him myself and he does great in certain meta's/match-up's
Recommendation: keep if you play control or plan to play control in the future, only DE if you exclusively play aggro or really really need the dust to fuel a deck that doesn't run him.
Captain Greenskin
Not used in the main meta atm. Played in some itterations of warrior control and in some niche rogue decks (mostly tempo) which can benefit a lot from the boost for something like an assassins blade. * Recommendations: If you know a deck you want to use him with you could keep him, namely the rogue tempo decks he's used in, also keep in mind that new weapons introduced in the future might make him valuable at some point. (TIP: if you plan on DEing him, get all the pirate cards first so that you can get your captain's parrots)
Deathwing
I. AM. POWER. INCARNATE.
Recommendations: doesn't fit into the meta however pops up now and then, fun to keep, DE if you need the dust though.
Gruul
I've only ever seen this card in Arena, like twice in over 1000 games of Hearthstone
Recommendations: DE unless you are building a 30 legendaries deck.
Harrison Jones
Pops up when the meta is weapon heavy
Recommendations: keep for when you need it, you won't need it often but it could come in helpful
Hogger
HAHA, no. Not even found in token decks, overall just not good enough to be included in any constructed deck atm. (Apparently used by some streamers in their shaman decks, could be half-decent vs some match-ups)
Recommendations: DE.
Illidan Stormrage
You are not prepared... for the 400 dust that this card will provide you with. Not used or usefull in any decks as his stats are quite poor for constructed and his 2/1's are to easy to clear and need to long of a set-up time to become good
Recommendation: DE.
King Mulka
Pops up in some aggro decks every now and then, gets some hipster points for it as well, stats aren't bad and it's good vs decks that can't ultilize the (mana costing) banana's
Recommendation: DE only if you really need the dust or if you only play control.
Leeeeeeeeeeeeeroy Jenkins
Pops up in... well.... Hunter aggro, Tempo Rogue, Miracle Rogue, Control Shaman, Mid-game Shaman, Zoolock, Handlock, Warrior aggro, any aggro deck on earth, 50% of all control decks on earth
Recommendation: Don't even mouse over the DE button, 10/10 would keep
Lorewalker Cho
Do you have friends to play Randuin Wrynn battles with?
Recommendation: If you're hipster: keep it, If you want 400 dust and don't have friends to play Randuin Wrynn battles with, DE. (Should be noted that the fun-factor of this card combined with the possible implications in future decks as a counter to spells is definitely possible and you should consider it if you decide to DE him, I am kind of itching more on the keeping side for that reason)
Malygos
Used in Malygod rogue, hipster-priest, Miracle druid and hipster-mage
Recommendation: if you really like the concept of bringing your opponent from 30 to 0 with a couple of 1/2 mana spells, keep him, only DE if you dislike fun or are on a really tight budget and really need those few extra rares for your Zoolock
Millhouse Manastorm
Coin > Millhouse > 0 Mana Sprint/Fireball/Pyro/Nourish/Anything is painful, not used in any decks besides TotalBiscuit's 30 Legend deck or for friendly fun matches in combo with Cho
Recommendation: DE.
Nat Pagle
It's a bit sad really, from 100% deck inclusion to 0% deck inclusion, used to be known as the most OP card in the game, currently it's just meh.
Recommendation: I hope you DE'd this when it gave 1600, otherwise, DE regardless.
Nozdormu
10/10 would use against a monkey playing hearthstone on the Ipad, not used in any decks at the moment.
Recommendation: DE.
Onyxia
Has some uses in warrior control and some hipster-decks (god forbid you make this work in Token-Druid or Shaman bloodlust decks)
Recommendation: Keep only if you have a very specific deck in mind with it, otherwise it's up to you to keep or DE depending on if you need the dust or not
Ragnaros the Firelord
Would recommend crafting a golden version if you play control a lot, present in over 80% of control decks, simply great for it's uses when played
Recommendation: 8/8 would keep / snipe, don't even think about DEing the golden version if you got it.
Sylvanas Windrunner
Used in some priest decks and pops up here and there in other control decks
Recommendation: Keeper if you play control, if exclusively aggro DE.
The Beast
The fact that this card isn't even played in hunter should say enough
Recommendation: DE.
The Black Knight
Sometimes this card is in over 80% of the decks in the meta, then he's in like 1%, then 80% again, great value overall vs any deck with taunts.
Recommendation: Always keep, solid overall
Tinkmaster Overspark
Used only by people who use it to counter miracle rogue by using it on a stealthed gadgetzan auctioneer, no use besides this purpose
Recommendation: DE.
Ysera
Pops up in control decks if the meta is slow enough to allow it, first legendary I got at the very start of beta, never decided to DE it, has uses in Druid / Priest especially
Recommendation: Keep if you play control, DE if you never plan to play really slow aggro decks
I will edit this list with your recommendations if I agree with them, also feel free to point out any spelling/grammatical errors, it was written in 15 minutes by a dutch dyslectic after all.
Josh
