Since I consider Opeth by far the greatest band MY ears have ever heard, it's difficult to come up with an arguement as to why they wouldn't be considered the greatest.
The obvious question is what needs to be considered to make a band the greatest in the world? I'll answer coming from one particular angle, and it's not about their talent, for I consider them 100% talented. The only thing I can come up with that could be thought of as not thinking of Opeth as the greatest band in the world is they are not known worldwide nor will they ever be. The Beatles, Elvis, Beethoven, for example....all known worldwide and written in the music history books. Regardless of how I feel about Opeth, they will never make "history". The select few of us who really know Opeth will remember them as great (or the greatest), but in the grander sense of the music industry and history, they will be nothing more than a blip on the radar screen. And it's a shame that the genre of metal just doesn't get the recognition that it should in the musical sense.