Discotheque is an awesome song, and my second favorite U2 album opener. Great video, too.
But it's up against my favorite U2 opener, and a top 5 U2 song for me. I just adore A Sort of Homecoming so much. There's so much discordant purity in this song, and Bono's performance going into the final verse (the wind will crack..." starting at 3:39 is maybe my favorite vocal moment in the catalog.
Fun fact - A Sort of Homecoming is (I think) the hardest U2 song to sing