If it were possible to make it to a festival in 2015, I think Northern Invasion would be in the running as my top pick. It's a one day show, it has some outstanding bands that I've never seen before, like Slipknot, Volbeat, Halestorm, The Pretty Reckless, Anthrax (the only band in the Big Four I've never seen live), and Young Guns.
That said, the 2015 edition of Rock on the Range, the 3 day concert on May 15, 16 & 17 at Columbus Crew Stadium in Ohio, has a pretty sweet lineup too. Once again, lots of bands I've never seen before. Of the first 7 bands listed, only Godsmack and Breaking Benjamin are bands I've seen before.
I'm flabbergasted and, of course, I have already found out it is a 9
hour drive from my house to Columbus, OH. Mark my words - someday, this will be a father / kids trip with Megan & Alex.