Critics hated the first few Led Zeppelin albums. In fact, they may have also hated the later ones, too. Kind of a controversial band for "borrowing" a lot of their riffs, but it's undeniable how well they have held up and musically proved the critics wrong.
Rush, Genesis, Emerson Lake and Palmer, and Yes were universally panned by critics. While Rush can be classified as metal, the other 3 produced music that critics called "art for art's sake." Errrr.... no. Ironically, ELP did an anthem that is played at today's sporting events all the time.
And since I mentioned metal, both Judas Priest and Iron Maiden have been criminally ignored by music writers. Maiden was a band I "came around" to, and I'm glad I did. I thought they were gimmicky, like a poor man's Kiss. When I listen to them with headphones without seeing all the crazy show, I get why they are so beloved.

---