The best one: A horse walks into a bar. The bartender looks at him and says 'why the long face?' The one people claim to be the best one: A baby seal walks into a club.
Two atoms walk into a bar. Atom one: How are you? Atom two: not too bad. Lost an electron today. Atom one: Oh, that's no good. Atom two: Oh, it's ok, I'm staying positive. BOOM TISH!! A neutron walks into a bar and orders a beer. He asks for the tab. The bartender looks at him and says "Oh, you're a neutron, no charge. BOOM TISH!! If you didn't laugh, there's something seriously wrong with you.