Goes to Scion of the Ur-Dragon!
Here's a 5 color General with flavor! The deck I faced off against was filled with dragons, a few card advantage engines, and recursion. If you've never thought of a dragon deck as a "tool box", seeing this in action is a real treat.
I kept thinking: "What's he going to turn his General into now?"
The best part, he can do it at instant speed. In my case, he threw a Nicol Bolas at me and I responded with Red Elemental Blast. He then responded by changing the Scion back into a non-blue permanent…Draco! I took a LOT of damage, but at least I wasn't losing my hand. A fair trade, and an awesome interaction. My brain got a workout trying to get around it.
The only time I saw him fail to find a good response was when he tried to get under an Austere Command set to destroy all creatures 4 mana cost or higher. There are few dragons to fit such a category, bar changelings.
The dragons are only online for 1 turn, then they go to the graveyard. Makeshift Mannequin, Animate Dead and company all work great for getting big fatties back into play on a more permanent basis.
One of the other players had a blast upgrading his Body Double every turn as well.
Just another reason why I love the format.