As we all know, there are plenty of cards that you're disappointed to see as your pack's rare card, like Prognostic Sphinx. I've opened 6 of those guys when M15 came out. :/ Luckily one of my friends liked those and traded me some actually useful cards for them.
Fast forward to Khans, and my friends opened up Hardened Scales, and when I saw it I fell in love and declared I would buy it for $5. Wasn't even foil. Didn't care, still don't, and its been the center piece of 4 of my recent deckbuilds in both standard and modern, winning me a few FNMs. Now that Dragons is out, my Hardened Scales deck has been tremendously boosted with Inspiring Call, Avatar of the Resolute, and Servant of the Scale. Swinging in for well over 60 damage among 3 creatures is a common occurence. And its a BUDGET deck. Still my favorite card.