New Belgium La Folie is generally regarded as one of the best. It's seasonal and hard to find though
Well, it's certainly one of the sourest/most acetic. Personally, I enjoy it a lot, but not everyone feels the same way.
I'm not sure what's available out there, but Jester King makes fantastic sours. I like Red Poppy from Lost Abbey a lot, and Gueuze Girardin, Gueuze La Fond Tradition and Mikkeller's spontaneous lambic series are my favorite Belgian sours (unfortunately no Cantillon or Drie Fonteinen out my way).