I already posted that I chose One, but let me say why:
Streets is their best live song, Bad is my favorite song of theirs, but One is their all around great song. It is for a couple reasons. For one, it is just an unbeleiveably great classic, sonically and lyrically on various levels. 2, it is undeniably U2 sounding, but not necessarily 80's or 90's U2. 3, it is the song that allowed U2 to successfully make the jump from the 80's to the 90's. Without it, who knows if U2 would have succeeded with AB and followed up with their 90's catalogue.