U2 18's listing was selected based on the songs that they deemed received the most radio airplay. simple as that. it was not a Best of in a best of U2 sense. It was a best of HITS and what's still played on the radio.
I have not heard Discotheque played on the radio, ever. I'm 26 years old, so there's some reference. I have never heard Hold Me thrill me on the radio.
I have heard each of the songs on U218 on the radio at some point in time. And I hear Beautiful Day, SBS, Streets, Vertigo, Sweetest Thing, New Year's Day and With or Without you by far the most frequently. Sometimes I'll hear One, but not as often as those others.
so in that regard, U218 is probably the one tracklisting they got right, and not because it's a comprehensive best of; it's just what they set out to do, get the most popular songs on one disc.