I'd say "Stairway To Heaven" but then 99,999,999 other people will probably say that, too, unless they're sick of it. Surprisingly, I first heard that song in 1971 and I've yet to say, "Enough already!!!".
But for this thread, my choice will be "Ramble On," from the album Led Zeppelin II. I've always loved John Paul Jones' bass playing on that one:
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