I've always liked the classic trick Out of this World. What I propose here is a version hat fascinated me since I had the idea a few years ago.

An Out of this World, with the deck face up! Yes, you read correctly!

You place face up one red card and one black card on the other side and you begin to show cards face up to the spectators so they place them as they like, leaving some on the red card and others on the black card. It doesn't matter if they're correctly placed! At the end, you're left with two groups on the table, mixed both with red and black faced cards.

But the magician slowly squares each group, and you can see the whole time till the second before the squaring is complete, that the cards are mixed. Even like that, when the magician returns to spread the packets, THE COLORS HAVE SEPARATED! On one side are all the blacks and on the other all the reds!

Additionally, it's automatic! What more could you want? An effect with 100% impact, and 0% ability necessary.