There's no argument that the worst halftime show in Super Bowl history was the Black Eyed Peas. The game itself was great that year with the Packers and Steelers but my god, that was just horrendous.
Other than that, I'd say I'm not real partial to the Janet-JT halftime. Not because of the music or anything, but because in response, they did old-timer's night for about 8 years straight with awful performances like the Who, Paul McCartney and the Stones. They were great in their prime, no doubt, but I changed the channel immediately when those came on, which is saying something because I enjoy watching the halftime shows.
Best? Michael. No argument.