Probably the most powerful gimmick deck built for the League has been the Orcish Artillery combination decks, which run about 10 copies of the Artillery (this was prior to the institution of a 6-card cap for uncommons), and a half dozen copies of Circle of Protection: Red
After all, Elpseth is literally on the cover of the MTG Oracle Deck box, and Elspeth, Suns Champion is an iconic Theros Planeswalker that played a pivotal role in Standard at the time
When Champions of Kamigawa came out, maybe a few players had a good time countering a huge spirit spell with it during the pre-release or in sealed pool games
The turn when you get to pull two or more phoenixes from your graveyard onto the battlefield attacking is usually backbreaking
Minsc & Boo is a powerful enough planeswalker to see some play in Legacy, and is still terrifying in Vintage Cube to this day
Hexproof, though, has proven to be a bit of a problem child