Kirriemuir [ sic ] in September used to be good. The last 3 times I went, it was crap, so I haven't been back. It all depends what you want from your Scottish Folk Festival.
If you're in the North-East and like unaccompanied singing, the Cullerlie weekend in memory of the late great Tam Reid is your best bet. 24/26 July.
Also in the North East - Stonehaven Folk Festival is one of the best in Scotland, [10/12 July ] and a "newcomer" at a mere 10 years old is the Portsoy "Haal" [ 5 /7 June ].
Hope this helps.