if I must pick a favorite I guess it would need to be pre-nerf owl raider, with lift+airstriker and double attack cones. she stopped the shrine rush combo's of UD focused around the fire bug, and de stealthed many of the flying stealth units that pestered us at the time, she was devastatingly powerful and very fast. all around I loved the number of different ways she could be used and the number of different things she did. if I'm restricted to runes that exist now....probably yeti ancient, I love his power turn mechanics and his durability.
Arrowsinger, because preparation is one of the coolest abilities every made and should never have been given to melee champs. Close second is Chained Fae - such a unique champion who does so much. As for general coolness, Hraken has to win hands down. Dude is a badass.
The original Lich King is my favorite. 8 range, Teleport, 7 speed and I have always liked the art work.
Jek the Silver General. I have a soft spot for the concept of "Eagle Knights" that exist in so many games. I tried to hard to make the Ospryans a thing in this game, but the developers never considered my ideas, unfortunately. SL/KF Beasts forever and always will be my favourite deck, since I made it viable.
I'd say probably the Myx Broodlord. He was always so difficult for folks to take down, and by the time they almost whittled his HP down to 0 I'd have him consume something and they'd have to do it all over again.
- I always loved Interrogator , that classic Frank Frazetta look. Champ never really worked but in my commons only deck she always had a spot to bring the pain.