Sylvanas Windrunner can be found in most Meta decks and is generally good in any deck. A solid 5/5 creature for 6 mana (nerfed) is not the greatest but the Deathrattle can be troublesome for opponents to deal with and forces the opponent to deal with the legendary. If the opponent does not have a targeted removal spell, usually the opponent will be forced to pump out small creatures because of the Deathrattle. It's best to send out Sylvanas when the opponent has more than 1 high costing creature on the board, so it can be quite situational. But in most cases Sylvanas is very effective placing, the creature card as one of the most popular legendries.
The ability to set a hero's health to 15 is very situational. Alexstrasza is very flexible to either destroy your opponent or to increase your own health in a near death situation outlasting your opponent. Freeze mages favour this card in the current Meta because of the possible direct damage with the Ice lance, Frostbolt combo after reducing your opponent’s health to 15 or comboed with ice block to outlast your opponent. Many other decks can use this card, changing the tide of the game, and potentially surprise your opponent when they think they have the upper hand.
You can expect at lot of control decks using Loatheb preventing opponents to use clears spells the next turn which can be great tempo usually giving the player an advantage. It's very effective against spell heavy decks and offers a good defence late game if the opponent has potentially has lethal through spell damage. If you also have the advantage in terms of board control with many creatures, Loatheb can prevent the opponent to play board clears such as, Starfall or Flame strike. It's a solid 5/5 for 5 mana placing ideal in terms of card value. Expect to Loatheb in most Meta games.
Dr.Boom has amazing value and damage potential the turn after. A 7/7 creature that summons 2 1/1 bots essentially makes it a 9/9 creature for 7 mana with the possibility of dealing 2-8 additional random damage. Most of the time this legendary is very troublesome for opponents to deal with. With 7 health and 2 1 health minions summoned will usually require the opponent to cast more than 2 spells to completely kill them off. The amount of damage potential is quite high making this one of the most used legendries out there.
Ysera is a difficult card to deal with. Having a hefty 12 health will usually require more than 2 spell cards to clear this creature despite the 9 mana cost. With only 4 attack damage puts Ysera in the safe zone against a Big Game Hunter and a priest's shadow: word of pain and death. The dream cards are also amazing at controlling the board, allowing for board clears, creature buffs, in addition to summoning two types of powerful creatures for a low cost value. Players have considered Ysera being too slow, however as the game progresses Ysera allows for so much power. Ysera is a legendary to be feared.
Ysera Dream cards
Another amazing neutral legendary where some players say this card is a bit over-powered if played correctly. Kel'thuazard is a difficult target to kill with a decent 8 health and 6 damage out of danger from infamous Big Game Hunter which has a battlecry that kills a targeted creature with 7+ damage. Paired with cards like Reincarnate, Ancestral Spirit, and Baron Rivendare you can potentially fill your board with Kel'thuazards making it near impossible for the opponent to deal with the situation.
Another situational card and at times players considered this to be a cheap card. Deathwing can completely turn the game around in just one turn clearing all enemy creatures. Since being the highest base damage and health creature in constructed and arena it's is very difficult for most classes to deal with Deathwing epically during the late game when players are more likely to have less cards. You could be in a situation where the enemy has the whole board full of powerful creatures with lethal damage next turn, and when they least expect it the tables completely turn. Play this card and expect people to rage quit unless they have removal spell or Big Game Hunter.
After the release Blackrock Mountain Emperor Thaurissan has been seen in most constructed decks and some players considered OP when comboed well. It is highly favoured for freeze mages and high burst combo decks. Reducing cards in your hand by 1 mana each turn is huge which opens up the possibility to create more plays to happen. It is ideal to have a large amount of cards in your hand to make card more effective and it is possible to have your hand cost no mana. I have the Power!
The Deathrattle RNG brings a lot more excitement to the game. In terms of value of the card is not that great, costing 8 mana for a 5/7. Players have considered Sneed's to be a bit slow so they are mainly used in control decks, although summoning another random legendary can be potentially game changing. When paired with other cards that synergises with the Deathrattle mechanic such as Baron Rivendare, Reincarnate, and Ancestral Spirit they allow you to constantly pump out random legendries for some amazing plays. Just watch out for hex, polymorph, and dispels.
Heres are examples of amazing combo plays with Sneed's Old Shredder
Ragnaros the Firelord is a solid 8 mana for 8/8 putting the legendary in the target area for value. Since Ragnaros can't an attack but deals it's attack damage at the end of the turn allows it to deal damage without taking damage itself making the legendary super-efficient. If opponents don't have a direct answer to Ragnarous such as a hex, polymorph, or Flame Lance it will be very troublesome to deal with which will usually cause the opponent to use up more than 2 minions or spells. The longer Ragnarous is on the table the more efficient it becomes. Making Ragnaros the Firelord one of the best legendries out there.