Gimme Some Lovin' - Spencer Davis Group. It's been covered at least 65 times; but nothing comes close to the original. THE definitive rock song!
Hallelujah has got to get a mention, I agree. Jeff Buckley's version on SACD gives me chills.
I didn't see any votes for Hey Joe. I have 13 versions of that, Black Cat Bone being my Fav, but Hendrix' is the classic.
I also have 15 versions of Morning Dew; but that's just me.
I'd also have to say that Janis Joplin singing Piece of My Heart never gets old; but probably the number one long lived hit is Mack The Knife by Bobby Darin.