There has been a bit of a battle recently, with Brewdog and Schorschbräu constantly topping each other's efforts: BrewDog Blog.
Currently it's a Schorschbräu Schorschbock 57% finis coronat opus, which comes in at 57.7%, beating Brewdog's latest effort: Schorschbräu Schorschbock 57% finis coronat opus.
I'd say the strongest commercially viable option (these 50% beers only ever have a few bottles in the batch and cost a silly amount) in the UK anyway, is Watt Dickie, also by Brewdog, which comes in at 35.0%: BrewDog UK.
EDIT:
Oops, I didn't know about Snake Venom, 67.5%: Brewmeister Beer.