Well, I supose my favourite Folk Festival has to be The Fylde Folk Festival, held near my home town. The festival is held every year (now in 27th year) at Fleetwood in Lancashire, England. Being so near to Blackpool there's a lot to do as well as the festival, so many people make a week of it (to the extent now that the Festival proper runs from Thurs-Sun, and the local council run folk evenings from the Sun prior to the Festival to the Festival)and also visit the local Folk Clubs.
The list of artists over the years reads like a who's who in Folk Music - and most of them have an entry on the Festival Site:
My second favourite is the Sidmouth Festival, held in July/August on the South Coast of England, the small sleepy village of Sidmouth comes alive for the week of the festival, with the music spilling out of the festival sites, to the street, promenade, pubs and nearly every corner - this year we're booked for 2 weeks - and will be down there for the full eclipse of the sun! I not sure which I'm looking forward to the most - the eclipse or the Festival!!
Take care and enjoy all the Festivals you get to - and if you get to the Fylde Folk Festival - say "Hello"