What would you include as the other 3?
I'm always going to think that PP is #1 no matter what, and MDF is the only other yearly fest I've been to. As far as festivals I've either been to or know about that have happened in the past, I had a fuckton of fun at Chicago Powerfest the year that Saturnus, Solitude Aeturnus, Novembers Doom, and Martyr played. I mean, Jesus Christ, that was a monster lineup, that's up there in terms of quality bands. Ozzfest, when it was still good (early 2000s) was a solid good time, for a rock show (hardly ever metal, even when it dabbled in things like Meshuggah and Dimmu). NEARfest, from what I understand, is a solid pick for great yearly fest.
I know that Milwaukee Metalfest has been the soure of many jeers in its recent incarnations, but I doubt anyone can deny the fact that that fest introduced so many quality bands to the States.
Heathen Crusade is a festival I wish hadn't bitten the bullet as quickly as it did. Maybe it was the location, the time of year, or maybe there aren't enough metalheads in America who realize how awesome those bands are that are the reason it ultimately became a financial failure, but god damn. I wish I could have gone to the ones that did happen, because I could add Thyrfing, Bal-Sagoth, Ancient Rites, Manegarm, Skyforger, among others to the list of bands I've seen. In other words, my life would be even more complete. Other bands to have made big appearances at this fest include the mighty Primordial, Moonsorrow, and Vreid. Holy shit.
And the one fest I wish I could have seen, even more than the unbelievable lineups of Heathen Crusade was the Day of the Equinox fest up in Canada. Green Carnation's North American debut complete with a full performance of "Light of Day, Day of Darkness," Agalloch actually playing a show on the eastern portion of their home continent, Novembers Doom who are awesome no matter what they play, Unexpect, a band that I have thankfully seen twice now, and Woods of Ypres--just an awesome band that blends what I love about Agalloch, Katatonia, and such together perfectly. That would have been the experience of a lifetime. Unfortunately, I had neither the money nor the time to attend.
The point is, when you negate past awesome fests, MDF and PPUSA are probably 1 and 2 still running, but certainly not the ONLY fests to be awesome.