I like mine chilled in a cooler of ice, and I eat it with a knife.
What made me ask the question was remembering my grandfather. He ate his cold, but salted it before eating. He's the reason I use a knife.
I know some use a fork, some use a spoon, and some (like my son) just bury their face into the slice.
What's your watermelon-style?
(In case you didn't know, to pick the best watermelon of the bunch, thump them and listen for one with the deepest hollow sound. It will be sweet and ripe.)