The best I can do based on memory and a bit of research are two examples of the purity mentioned in the show:
1) In a flashback scene at the start of Season 4, episode 1 titled "Box Cutter", Gale tells Gus that his product is 96% pure, but that whomever makes the "blue stuff" has achieved a purity of 99%.
http://www.imdb.com/title/tt1683084/synopsis
2) In the opening scene to the Season 5, episode 7 episode titled "Say My Name", Walter specifically states that his product is 99.1% pure compared to the other group's 70% purity.
http://en.wikiquote.org/wiki/Breaking_Bad#Say_My_Name_.5B5.07.5D
As such, the highest quality I can recall being referenced specifically is 99.1%.