My favourite band of all time is Mercyful Fate, with King Diamond being a fairly close second. Rounding out the top 5 would be Black Sabbath, Judas Priest and Iron Maiden.
But "the greatest" and "my favourite" are two very different things. Black Sabbath, Judas Priest and Iron Maiden are the greatest Heavy Metal bands of all time. It's not up to me or you to decide that, it's simply a fact. They're the three most innovative, most influential and most iconic Heavy Metal bands in the history of the genre, end of story. Metallica released some good records, possibly even a great one, but they've never been consistent in terms of quality or style and they haven't released an album that might even be considered mediocre for almost 30 years, never mind a great record, so to suggest that they deserve to be anywhere near the top of the list is laughable. Every band has one or two clunkers - Sabbath, Priest and Maiden included, but Metallica doesn't have a legacy anywhere near as legendary.
To propose that Pantera might somehow qualify to make it on to the list of even the 100 greatest metal bands of all time is just a ridiculous statement. They were a shitty glam band for years, then decided to brand themselves as some sort of Texas hardcore - thrash hybrid, a formula that served them well on "Cowboys From Hell", but got old very quickly. All their songs sound the same, they showed zero progression or growth musically and their records are rubbish compared to heaps of other bands.
Metallica and Pantera are safe, friendly, approachable, non-threatening bands whose fan base primarily consists of blokes under 25 who take comfort in the easily digestible and simplistic songs they churn out, chaps who aren't mature enough to appreciate the talent demonstrated by bands like Amorphis for example.
To be "the greatest", you can't just be a band that a few people like. You've got to be a band that defines an era, that spurred on 10,000 other bands to follow in your footsteps, a band that other musicians point to and say "We wouldn't be here if it weren't for them."
Metal would be doing just as well as it is now if Metallica or Pantera had never existed. But without the Big Three, it wouldn't have gone anywhere.