Crows. They're incredibly intelligent and seem to have settled on using that intelligence almost solely to be little mischief gremlins. They're the only kind of bird that seems to understand that my cat can't get to them through the window. I watch them work in pairs to steal food from seagulls - one goes in for an obvious attempt, all the seagulls chase it off, the other grabs a bite while the gulls are distracted, and then they swap roles. I've even seen one actively dipping chips (fries) into a pot of ketchup. In winter you can very occasionally spot them playing in the snow. They'll intentionally slide down a snowy car windshield or roof, then fly back up to the top to do it again.

I volunteered at a greyhound rescue shelter for a little while, where there was a rook (very close relative to crows, distinguishable by their paler beaks). He had been hit by a car years beforehand and the owners of the shelter found him at the side of the road. They took him to a vet, who had to amputate one of his wings, so obviously he couldn't live wild any more. He was instead having a comfy life hanging out at the shelter. Totally unafraid of the humans or the dogs. When I sat down to have my lunch next to him he would yell at me to share a biscuit with him.