I'm wondering what people's favourite cards are? This could be purely based on strength of the card, but I am more thinking along the lines of "ha, cool" or "nice sound effect!"
My favourite three are:
Tinkmaster Overspark: a 3 mana 2/2 guy, who transforms a minion into either a 1/1 squirrel or a 5/5 Devilsaur. He seemed terrible at first, but I do like the coin flip cards, so I used him in my paladin deck to see if he can turn my 1/1 recruits into a 5/5. During play, I realized that it's also a neat trick against the opponent: this little fellow saved my bacon on many occasions - a rogue placed a guy and then did that combo thing for +4 attack. All of a sudden I faced an early-ish 5/3 guy. Turning him into a 5/5 wouldn't be a big gain for my opponent, but fortunately he turned into the 1/1 squirrel ("this time for sure!"). In another game I morphed an opponent's 8/8 guy, which would have been a gain, even if he had just turned into a 5/5 guy. Plus, the squirrel sound is too cute :D
Hogger: a 6 mana 4/4 guy, who summons a 2/2 gnoll with taunt at the end of each of your turns. He's quite powerful, which is why Hogger is in all of the decks that I build. But that aside, I love his "Mwahaha arooo!" sound effect when he is played :)
"Just me and Mr. Bitey": I actually forgot the card's name, but that 2/4 gnome with the 2/1 mechanical dragon is just too funny. I actually called one of my early decks "Mr. Bitey", but nowaday I don't use the card (outside arena) anymore.
So, what are your favourites and why?