My standard answer for these threads is “Whole Lotta Love”, Led Zeppelin II. I do have quite a few honorable mentions but I’ll respect with the “one answer only” premise of the thread.
Honestly, I kind of prefer it when the opening song isn't the best on the album. I think I'd rather a stunning closer or a monster of a song later in the album. With that said, I do really love the way this song sets the tone of the album, the rhythm is instantly catchy and it keeps your attention while it builds over the next two minutes into a stunningly catchy danceable art-pop gem reminiscent of Remain in Light era Talking Heads (Which I nearly picked for it's opener Born Under Punches). LCD Soundsystem - Get Innocuous
