It’s hard to beat
Of course, you should get all of their albums, but especially Arrival->The Visitors. Absolutely timeless. Great intro to their catalog and hits most of the hits, but of course, there is More ABBA Gold as well to continue that story plus through an previously unreleased gem in the mix...
I grew up with them, remember their break up as couples as a child and their break up as a band as well ... then came the haters... never understood why they had such a backlash, but by the early 90’s they were being recognized once again for their timeless perfection. I loved them all the while and still do!