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.
Both are worthy cards, who wouldn't like them....... but the one that was absolutely fun to play was.......
Shifting Sky Enchantment 2U (Uncommon)
As Shifting Sky comes into play, choose a color.
All nonland permanents are the chosen color.
Just have creatures with protection from one color and choose that color.
I liked to play creatures with Pro black and drop this to make them invincible and unstoppable....... Granted it wasn't a definite game winner, but it was fun.
Sorry! Nothing got sent out last week. I think was expecting someone else to send out the "Ballot"....... Actually, I was expecting Spidey to do it, but then remembered "He's not on the committee....."