The premise? "Number one songs and songs from number one albums". The reality; a repackaged ABBA Gold with three substitutions.
Unfortunately, although the three replacement songs are catchy and fun, two of ABBA's very best songs ("Lay All Your Love On Me" and "Thank You For The Music") have been lost. Worse, it still doesn't include Gold's only serious omission, "The Day Before You Came".
If you're considering the 2CD special edition of "Number Ones", bear in mind that Disc 2 is just a mish-mash of album tracks. (Some are excellent, but you may as well buy the original albums.) And it still doesn't include the missing songs above.
My advice? If you want a single-disc compilation of the very best ABBA hits, buy "ABBA Gold".
For little more, the double-CD "ABBA: The Definitive Collection" includes *all* the songs on Gold, plus lesser-known classics. Even with a couple of ropey early singles near the start (the tracks being ordered by date), it's definitely the best ABBA compilation. Both this and "Gold" are still readily available through Amazon.
The music is still 5 star material... but as a slightly inferior and totally pointless rehash of ABBA Gold, "Number Ones" really doesn't deserve that much.