Of course it's all subjective, but I think in general, there's a way to judge by longevity and also just how many singles did well.
A vast number of pop bands and singers manage a top ten hit, even a number 1, (and I know the charts are mostly irrelevant nowadays), but ABBA and most people who are generally considered to be 'good' have a solid catalog of music that's well known and successful.
I imagine you could ask a dozen teenagers nowadays and ten of them could name 3 Abba singles. Try that with Bieber or the Jonas Brothers in 30 years. I'd suggest that Britney Spears might be remembered a couple of decades on, while I can't stand her style of music, she certainly had some solid singles remembered and loved by many, and is a recognised name and face, like Abba, whereas Pink, Aguilera, and many others may well be forgotten in 5 years.
Looking back to rock, in the grunge period, Nirvana are legendary, but I'd personally rate both Soundgarden and the Stone Temple Pilots above them (opinion is all), but I'd still give Nirvana the credit for making the sound almost mainstream, something accessible and popular, and producing catchy tunes, despite feeling that the other two bands were better musically.