I wouldn't nominate a card I'd never use, and Planeshift really seems to kind of suck. It was the set I was most disappointed with when it came out. There are some cards that I'd definitely use, like Quirion Dryad or Dromar's Charm. But they don't stand out as "casual card hall of fame" candidates. There are a handful of cards that do. I nominate Terminate.
A lot of my opponents liked black and red decks back then, and Terminate was a card that really helped make them good. When Planeshift came out, I only had one Crystalline Sliver and so my Sliver Queens were common targets for Terminate. But I used the card to my own advantage too. Terminate is one of the best gold instants and one of the best creature removal spells ever. And it was well-balanced too.