This one isn't tough, though it is. Tom Waits' "Downtown Train" has been covered by everyone and his mother. Or at least by Everything But the Girl.
Actually, the most successful cover is by Rod Stewart, I believe. Still Bob Seger, Patty Smyth (minus Scandal) and any number of others.
So, who is covering who covering who, exactly?
I've opted to go with the one cover on Mary Chapin Carpenter's debut disk.
Technically, Carpenter's version isn't a cover of a cover I suppose. Her was released in 1987 (Waits originally released in '85). Smyth's came out in '87. Stewart and Seger two years after that.
I'd say I'm quibbling, but in reality - none of you care.