Garth Wood-Anthony Mundine II was a really rough fight to watch. That was only 10 rounds though, so it'd be behind some of the others mentioned plus Klitschko-Ibragimov.
Virtually any fight featuring that cheating slug John Ruiz and any decent fighter - because you know he'd be resorting to excessive holding and other illegal tactics. If I had to nominate one, I suppose I'd go with Ruiz-Rahman
I personally can't watch anything with Devon 'The Hugger' Alexander. Maidana v Alexander was too frustrating to watch. Too many **** fights to name...