Malygos: Giant spell buff, could also make a sturdy tank if you have cards to make him a Taunt. Pretty much all the Dragons can do that if you want.
Alexstrasza: She’s a very popular card and for a good reason. Lets say it’s late in the match. But you weren’t really able to do a lot of damage to your opponent. Like a Warrior stacked in armor. You have her attack your opponent and their health goes down to 15. She can also be used as last minute heal. Putting you to level 15 and hopefully out of the lethal zone for another round.
Nozdormu: I love this card and yet it’s really tricky to get him into a deck. He makes the turns go faster so the opponent has to rush. But most of the time your opponent will have a card that can take Nozdormu out. So you’ll mostly find him in Gimmick decks.
Onyxia: I don’t think I have ever used this card. But it is a fun idea to just summon her and all her little whelps. Those whelps would probably be very annoying if the opponent has no AoE cards.
Ysera: Another popular Aspect. Since she gives you special cards. So even if you take her out in one turn, the opponent will at least get one special card.
Deathwing: Deathwing is pretty much a giant middle finger to the opponent.