In terms of collectibility or popularity that is. I believe sometime back in the mid-to-late 80s TIME magazine declared that U2 may be the second-most popular band in music history after The Beatles.
I think this may still indeed be true when looking at the entire scope of popular music history. Their world influence cannot be denied. Now, of course, I think the Stones are right there. However, I don't think Bono will ever pass Mick as an icon, although Bono is hard to top as a humanitarian.
So who comes in second after The Beatles? There will be the Zeppelin fans. But their popular influence simply does not touch U2. It's not close.
Oh...but let's not forget One Direction. (LOL!!!)