Where/ when/ why
Of all the festivals I have been to, for the performances, I would say Orus Festival in Mexico City in 2015 was the best. It was not the bands that made it better, they are almost always the same, but the people who worked in the venue where it was held wore elaborate costumes and fire danced amazingly.
Amphi Fest has the best actual venue because it's next to a river, inside and outside and it's pretty clean.
WGT is just different because it is a whole city.
...as far as the actual festival though, Orus 2015 was the best