I will happily acknowledge that I'm a big ABBA fan as well, possibly more so than my missus and her various mates who like Mamma Mia (stage or film). They try to disagree, but I fail to see how someone who would prefer to listen to the Mamma Mia soundtrack album can claim bigger ABBA-fandom than someone who regularly listens to the albums made by ABBA themselves :lol:
Not sure whether they're the best pop group ever, though?
That's probably quantifiable (with a few punch-ups over the sources of and the validity of statistics) - the group with the most units shifted, radio/media plays, concert tickets, etc, so far, probably qualifies as the best pop group ever - it's a popularity contest. So if it turns out to be U2, for example, I shall probably be sick, but I won't be able to argue with it.
If you take the word "pop" off the front, then we're into a whole different ballgame and most folks (on here, anyway) will agree you can't really say who is "best" out of different acts who are pretty much chalk and cheese.