Depends what is less silly, judging bands based on opinions or no...based on opinions, any band can be the best, even One Direction. I rather judge based on things like influency, consistency and many other things. Of course you can have your favorite bands, but they're being along the greatest only because it's one of your favorites it's another thing.
EDIT: Also, the greatest bands club thing, i'm talking something like TOP 50 or 100 bands, not 1000 or 2000.
Sure, someone can THINK that the best band ever is One Direction. In fact, I know people who do. Now, that opinion can be more or less justified (in this case, less, in the case of, say, Pink Floyd, more), but it's still an opinion.
You seem to be out to create a standard for greatest band of all time that will eventually lead to a conclusion that everyone can agree on. The only way to do that, since no one has devised a method to evaluate the quality of music objectively, would be to use either sales or some formula involving the sales of bands they've influenced.
I think we should use the word "greatest" to refer to something closer to the actual meaning of the word "great." Even if it means we can't state definitively that one band is the greatest.
Note: I think that my method doesn't eliminate the distinction between favorite band and great band. I might consider a band to be a great band judging on the quality of their musicianship and songwriting, but I may not personally care too much for it. For me, Zep could be a limited example of the former (I think they're overrated, but still a good band. However, I don't enjoy them very much aside from a few songs). Similarly, I might recognize the mediocrity of a band's musicianship or songwriting but there could be something about that them that makes them one of my favorites, even if I don't consider them to be that great (I really like Steve Harris's solo band though they really aren't that fantastic of a band).
A good metric for greatest band could be what I suggested: musicianship and songwriting, perhaps measured over a period of time (to determine consistency). I think DT is the greatest band in all these respects. They have an exceedingly high level of musicianship as well as superb songwriting, and DT12 is every bit as excellent in these respects as
Awake. All of these traits, they have more of than any other band of which I know. That's why I think they're the greatest band ever.