Tons of casual players were already playing with huge decks, so this card gave them a win condition based on unique deck building skills.
Even though a few decks were built to exploit this card, it was the champion of the casual mountain of card deck builders.
Atogatog (It's an Atogatog)
Chatter of the Squirrel (Squirrel #1)
Squirrel Nest (Squirrel #2)
Squirrel Mob (Squirrel #51)
Graceful Antelope (It has plainswalk...)
Holistic Wisdom (I dunno, but at the time, this card was always in my casual decks that had green in them) Iridescent Angel (my selection)
Any of these cards (to me) would be suitable candidates for nomination. However, Iridescent Angel had something that nothing else had considered before: Protection from all Colors. Not "Cannot be Targeted by Spells or Abilities". Protection from all Colors.
On top of that, it wasn't too entirely broken. At a hefty cost of 5UW, you'd have to be playing for the long game (another casual trait in my opinion), but with it's high cost, it wasn't too easy to Upheaval and float mana for it. It get my nomination.