I think with the 'Best Ofs' U2 were trying to put themselves in the headspace of the fan, and not so much the hardcore fan, and U2 aren't very good at that headspace.
In fact as hardcore fans most of us wish they would never do that, but the truth is sometimes you have to. I almost think they should have left this type of decision up to someone else, maybe Gavin and a few others and then come in and OK the picks.
I think U2 like many other bands, feel very differently about their songs than the fans do. And we may never get the truth about songs as long as we live.
Some songs as brilliant as they are may be a thorn in the band's side because of how much struggle they had to record it, or because personally they never fealt they quite nailed it in the studio.
Others they may absolutely love and it's on the bottom of lists for many fans.
We just don't know. So there may be personal connections to songs, good or bad that may make it impossible for members of the band to look at them objectively.
Some bands come out and tell these things years later and some we'll never know. I personally know a band that recorded a song that is almost universally loved, and for ten years they've been playing it every night of every tour, and almost every member of the band absolutely hates it. This same band has a song that is their personal favorite and they believe was one of their heights creatively and it receives one of the most underwhelming responses of the tour, much to the band's dismay.
So we just never know.