It is a bit of an odd title: very subjective for attenders, and very bias-prone for organisers. Those of us who run festivals do strive to make it "best ever" each year and keep raising the bar, and the only way you can do this is to be self-critical (after you've wallowed in a few pats on the back in the immediate aftermath of the event) If you're into doing feedback forms, "economic impact assessments" and the rest, it's worth reading all the comments, even if some are quite laughable and totally unrealistic, but others are genuinely useful and can lead to improvements in subsequent years.
So I'd say Linlithgow Folk Festival - may not be MOST improved as it's already so good in my totally unbiased (ahem!) opinion, but ever improving if I have anything to do with it! Dates for 2013 are Sept 12th-15th.