I've just heard Mark Kermode's SATC2-induced tirade on their end-of-year film review thing and it struck me, it might be one of my favourite rants of all time. I love the way he starts out determined not to get angry which leads to all the anger boiling up inside him, and a subsequent explosion of rage.
http://www.youtube.com/watch?v=uHeQeHstrsc

What are everyone else's favourite rants?

Keegan comes to mind, as does Jeff Stelling, although that was too calculated and together to be a true rant.