Difficult one to answer. Probably the most widely popular is Waterloo. The most critically acclaimed would be The Visitors. But I think my personal favourite is Super Trouper. I don't actually rate the title song much, nor the equally well known Winner Takes It All. But it does have a lot of my other favourites on it.
Tell you what, as you're so interested, I'll give you my list of top ten personal Abba favourites (in order of preference of course).
I do I do I do I do I do
Our Last Summer
Rock and roll band
(The first half of Me and Bobby's brother and the last 45 seconds of People Need Love would have made the list if the rest of the songs were as good as the good bits.)