Ysera: My most favorite card in all aspects (entrance, animation, voice, 4/12 is the best stats you can get in the game), Nightmare and Ysera Awakens have won me too many games, its also the reason I play Dragon Deck and get me my first time Legend.
Velen's Chosen: Best card to buff your minion, will win you a trade most of the time and +1 Magic damage combined with Holy Nova.
Aldor Peacekeeper: My favorite low cost and most effective card, that feeling when you turn the Giant into an ant.
Nefarian: I like it as a Dragon race, the RNG, good value for my Warrior deck.
Sylvanas Windrunner:The best designed card in the game imo... I love seeing my opponent try to play around her and then having their best minion stolen anyway or even better using Shadowflame on her... Here enjoy 5 AOE damage while I gladly take control of the your only fatty that survived the slaughtering....
Azure Drake:I don't why I like this card... It just caught my eye since the first time I saw it and it was one of the first cards I crafted... Now I proudly own two of these in full Gold,they look sooooo good.....
Lord Jaraxxus:Owning control matchups since 2013... How can you not love him?...
Deathwing:His entrance is soo epic,also he reminds me of my favourite Yu-Gi-Oh card...
Coghammer:This is probably cause all Arena runs in which I drafted this card ended up going well.... It was the MVP of my first 12 wins run so I grew fond to it... Never opened it in a pack,played an Arena in which I drafted two and swore that if it ended with 12 wins I would've crafted it... Guess what happened next....
EDIT: From top to bottom as most favorite to least favorite.
Yogg-Saron, Hope's End - This card literally makes both players lose control of the game. How it makes and breaks matches makes it seem like the fairest RNG there is, because both players can get screwed by him.
Elise Starseeker - I used to try to put her in every deck I had (and it is still tempting). Her potential is so large and unpredictable! My favorite matches are when both me and my opponent find the Golden Monkey and its a battle royal of all kinds of Legendaries that would otherwise never see play.
Iron Juggernaut - The mines are the coolest lethals, period. I fell in love with it ever since it won me my first match (I still don't have it in my collection, BTW) and it always saddens me to know it never found its place in the meta.
Piloted Sky Golem - While the Shredder got stale pretty quickly due to how often I still see it, this guy (or machine) always has a surprise in store. And how it sometimes drops a Flamewreathed Faceless makes it slightly better than before. Plus, there are tons of interesting 4 drops in this game that deserve more play.
Unstable Portal - Used to be my favorite, but now that I play against it, I know how broken and frustrating it can be. However, it is one of the few ways for new players to learn new strategies with cards that they would never otherwise get any time soon. I always liked it just for that chance of getting a legendary and to pretend that I was a veteran.
Start of Year: Provoke the failure of 3 expansions, force nerfs on otherwise balanced cards, bring deckbuilding to an all-time low and get rotated one year earlier for being such a threat to the game's health. - Genn and Baku's historical entry on the White Book of Shit Design, shortly before retiring unpunished
Ysera: First legendary I ever crafted, the dream cards are just so fun and give great value. Close to a must in any Dragon deck and one of the most interestingly designed cards in the game.
Archmage Antonidas: The first card I ever tried to build a deck around, his ability is both powerful and fun and it's really satisfying when you manage to pull off a game winning combo with him.
Duplicate: I love the secret mechanic and duplicate was probably my favorite in the old control decks I messed around with, it's a shame that in standard as it stands most Mage secrets are not particularly viable due to not having a reliable way of getting them on the field early enough without a huge loss in tempo.
Coldarra Drake: Another card I messed around with a lot due to it being potentially really interesting but sadly never managed to make it work consistently.
Sylvanas Windrunner: Has always been one of the best legendary cards in the game but only when played right. It's not a Boom or a Tyrion which will almost always be a good play due to their borderline brokeness, you have to pick your moment which is what makes it interesting.
Lord Jaraxxus - I really tremble when I play this card. Is so incredibly powerful when you play him in the right moment. And when I'm playing against him, I feel like a cat moving his tail with eyes on the prey in signal of being ready to hunt. I feel like this card is one of those cards that always generates fun games. Also one of my first legendaries.
Azure Drake - This card is so nice to look at and very versatile. It was the first rare I crafted so I really appreciate it.
Renounce Darkness - One of the coolest cards ever made in Hearthstone. Its effect, the animation, the RNG, the gachiGASM face and having this card golden makes this card worth pure gold.
Lord Jaraxxus - I don't even know how to express how amazing this card is. This card represents the power of Eredar Lord with amazing emotes , so truhly that you tremble when you play this , yet this card has good function and place in game as key control card.
Tirion Fordring - Whenever I play this card I feel the true power of light and the spirit of my favourite paladin character. I really pray to the light ,when I need to top deck him and he almost always comes. It's one of my first legedaries and had a lots of fun with this card an my noobie mech paladin.
Pyroblast - Real demonstration of Mage's power .I have some many good memories about this card and my old Mech Mage deck that used to run it . That feeling when you just "blast" your opponent for 10 is insanely amazing.
Chromaggus A Dragon that will give a copy of every card you draw. Sadly people destroy this card quickly when you play it. I play this mostly in a Dragon deck with Dragon Lord in it. When Dragon Lord is destroyed, it will put all dragons you have in your hand on the table. Should you have played Duplicate before that, the chance they will go after Chromaggus is very slim. But even if you don't play him in a Dragon deck, and you can get some duplicated cards, it could make you win the game. For instance double Ragnaros, or what to think of multiple Faceless Manipulators, Reno's or whatever. A fun card.
Chromaggus + PW Shield + PW Shield once gave me 4 times Mind Control. Was a fun game..well..for me, I don't think the CW liked it as much.
I agree, I rarely play Chromaggus on turn 8 itself unless I have noting else to play and need to gain board control, turn 9 Chromaggus with PW Shield is ideal way to gain the full value.
5) Doomsayer With the lack of AOE Spells Doomsayer is the only neutral card thats helping to keep Aggro decks at bay.
4) Reno Jackson This card is absoloutley brilliant. Because of Reno Jackson so many control decks are now viable because they have Heal to keep them in the game. I love the effect and the idea of the card... I just love Reno!!!
3) Sir Finley Mrrgglton/Emperor Thaurissan Not only does Sir Finley Mrrgglton have the best name and emotes in all of hearthstone but his effect is so unique. This deck gives Classes like Warrior and Shaman a chance to become super aggresive, and helps classes like Hunter become more controly. And of course the absolute king of combo decks Emperor Thaurissan. This card is just so much fun to play and offers so many unique combo opportuinties and decklists. This is a card that I really wish was a part of the Basic Set.
2) Loatheb Just a brilliantly desigend card. Not only does it take strategy to know when to play the card but the effect is amazing and balanced. Truly one of Blizzards best Creations.
1) Yogg-Saron, Hope's End Has to be just about the most amazing card ever made. The effect is just so satisfying to watch that this card alone has just improved my hearthstone experience tenfold!