WOWY, One, Pride, ISHFWILF, probably in that order.
Then Beautiful Day, Vertigo, NYD, SBS, and Mysterious Ways, again probably in that order. Desire and AIWIY would be the two remaining contenders to round out the "most popular" top ten.
After writing this, I had a look to see what the all-time top ten is on last.fm. Nine of the eleven songs I named above make it - Streets is there instead of AIWIY and Desire. Keep in mind this is based on over a decade of listens by hardcore fans and casuals alike, across the globe:
With or Without You: 983007 plays
Beautiful Day: 957124
One: 791389
Sunday Bloody Sunday: 686778
I Still Haven't Found What I'm Looking For: 677359
Vertigo: 596097
Pride (In the Name of Love): 557614
Where the Streets Have No Name: 534746
New Year's Day: 473653
Mysterious Ways: 425193
Also note that some statistics are distorted by people entering different titles. For example, there are an additional 70,824 listens for "With or Without You - remaster" and almost 70,000 combined for "Pride" and "Pride (In the Name of Love) - remastered". This doesn't take into account all the listens to tracks with "live" in the title field either.
Dig deeper here:
https://www.last.fm/music/U2/+tracks
Some curiosities I've noticed from the all-time chart: Crumbs, One Step Closer, and Yahweh are all above Acrobat, possibly reflecting that last.fm (as Audioscrobbler) first became popular when HTDAAB was the new album; Stand Up Comedy somehow has more listeners than Ultra Violet; Christmas (Baby Please Come Home) outdoes gems like A Sort of Homecoming, Zooropa, and Gone. But in better news Love Comes Tumbling beats Miss Sarajevo!