Leonard and Penny (The Big Bang Theory)
I have never found a canon couple with less chemistry EVER on a television show, but Leonard and Penny take the cake. Of course, it doesn't help that I don't think they work on any other romantic level either. In addition to no chemistry, there is no believability to the pairing. Nada, zilch, zip, zero. The show is so very much better when they are just friends. They work as friends, but whenever romance (or, let's get real, in their case ... sex) enters the equation, things become uncomfortable and awkward (but not in the intended, haha-funny way of Michael Scott on The Office). Johnny Galecki and Kaley Cuoco do not have a lick of romantic/sexual chemistry at all, I don't care if they dated in real life. They're the rule that proves that those involved in real life have no chemistry on screen. Because they don't; they really, really don't.