The best Festival is the one that suits your psychie. Whitby is a week and that can be a lot of one thing. Sidmouth ditto - and Sidders in it's present form (can't speak for previous iterations) was enough (on the basis of 2.5 days) to convince a workaholic gardener to forgo a week's work in the height of the season, and Joy is not a musician (yet) but a keen dancer. I have always been wary of a whole week - preferring to go to many weekends over the year, personal taste I guess.
Bromyard is good for sessions and conserts and dances.

But best - for me? Probably Upton-u-Severn because I have been loyal to it since it's inception - pick yer reasons.