Pantera was and is heavy metal. I would say metallica was the same. Their latest album was nu-metal and I don't know where I would put everything after the black album through garage inc.

Was pantera the heaviest at its time...

Death put out "Scream bloody gore" in 1987, ushering the dawn of death metal. Granted death was and still is 'underground' in the mainstreams eyes, but they were out and starting a movement. Pantera's "Cowboys from Hell" came out in 1987. I think Early Death is at least comperable in "heavyness" to Pantera if not heavier. I'm sure their are other better examples, but no Pantera wasn't the heaviest thing at the time, but it was definately the heaviest that mainstream saw. IMHO.