The last band you want to see on each day. And after they're done, you sit in the grass and talk about how amazing it was before a long walk to your car/campground.
Friday - Aphex Twin
Saturday - Up in the air, there's nobody who really stands out as a perfect ender, but I guess I'll go with Portishead.
Sunday - Justice. Bonus would be with a ton of Ed Banger cameos during their set. I've been waiting for a fucking while to see them and they are #1 on my list for this year.