Any metal groups (or any music group) you thought was really great once upon a time, you wouldn't listen to now?
That's kind of a tough question to answer. I don't
think there are any that fit what your asking exactly; that said, there are a few that are near to the mark. Metallica is a band that I've kinda turned a cold shoulder to, but I still listen to them on occasion. Pantera I still like, just I don't give them as much attention as I once did... then again, they were never a "favorite". Then there's also Mercyful Fate, which I still love, just not as much as I used to.
Megadeth though... they've really slipped. From being my all-time favorite metal band, they're still in my Top 10 but I don't listen to them half as much anymore. A lot of it has to do with basically coming to the conclusion that Rust In Peace has crap production. I still listen to them, but not as much as Maiden, Sabaton, Death (which is actually another band that's slipped but it kinda comes and goes anyway), or Buckethead.