If I were to vote based on Most Clever Use of the Challenge, my vote goes to:
Rattler of Det
While the challenge itself isn't a particularly clever challenge, I feel like this submission found a way to *make* it clever (I lol'd) by making a card that actually referenced the challenge rather than just obeying the limitation.
Honorable mentions (ones I'd vote for if I'm voting based on plausibility/balance/gameplay/flavor):
Muradin Bronzebeard - Great lore character and perfect effect for Muradin. I imagine him coming down turn 9, smashing his hammer 'Thor'-style, dealing damage to enemy minions and gaining some armor. It's a good AoE effect for a class that has a lot of AoE but it's a 1-of and perfect for the warrior class IMO.
Pyramid - great flavor, unique effect, somewhat difficult to trigger but a nice reward if done. Definitely not game-breaking. [Worth noting that I sometimes catch myself feeling biased/skewed towards favoring the first card in the top-left of these lists because it's the first card one reads... but I do my best to be objective and this time it definitely earns its place.]