I've been on a cover song kick again lately and polled friends about their favorites. (I put some of my favorites on this playlist from Spotify.)
The best cover songs give a nod to the original but take it in a different direction. My favorites are songs where the original is beautifully written but sung by someone whose voice . . . leaves something to be desired (Bob Dylan, Neil Young [sorry fans]), subsequently sung by someone who is just as masterful a musician with an expressive, full-range voice (Shawn Colvin, Cassandra Wilson).
Lots of cover songs are fun as novelties. Tori Amos covering "Smells Like Teen Spirit," Richard Thompson's "Oops, I Did It Again." The danger, however, is that these covers can cross over into, well, not being nice. And with any novelty, they don't always wear so well.
Songs the listener feels nostalgic about can go either way: novelty or classic. Totally subjective. Despite its gimmicky nature, I love Shonen Knife's version of "Top of the World" both because I grew up listening to the Carpenters and I love the joy in Shonen Knife's delivery.
My friend Beek asked "does it count if they are songs that were recorded by another artist first but not commercially successful?" My answer: It counts if I like it. My compilation includes the Mary Lou Lord cover of Freedy Johnston's "The Lucky One" and Feist's cover of Ron Sexsmith's "Secret Heart," neither of which, to my knowledge, was commercially successful. But both originals are favorites of mine.
Bonuses on my compilation are songs that have been frequent entries on mix tapes. You know who you are, songs.