Anyway I think in general, statements like "________ (insert this or that band) is the BEST band of all time" are PRETENTIOUS and they could cause a strong argument with many music fans who disagree. If you say "This or that band is my FAVORITE band of all time" nearly nobody would argue with you for that.
For the most part, this is no principled distinction between "Mumbling Carcass is the BEST BAND EVER!!!" and "Mumbling Carcass is my FAVORITE BAND EVER!!!" If that's why you're saying that the former sort of statement is "pretentious" (i.e., because it is a statement of opinion pretending to be a statement of objective fact), then I agree. Otherwise, I'm not sure what you're talking about.
Is Rush one of the bands with most rabid fans? If they are, why?
If you can explain to me how to measure rabidity, I'll be happy to answer this question. Absent such an explanation, I will simply agree that a significant percentage of Rush fans are passionate about the band.
You've obviously never seen Tarja fans go at it with other Nightwish fans.
Since TAC apparently overlooked this: I've never seen it or even heard of the two bands you mentioned.