Growing up with Sesame Street, I always liked David (Northern Calloway) and Gordon (Roscoe Orman) the best. Susan (Loretta Lynn) was also awesome. Check out her duet with Roscoe, "Living Hand in Hand" from the Bert and Ernie Sing-Along LP (1975). It could easily have been a contemporary hit, not just a "kid's song" (of course, many of SS's songs transcend the kid's song stereotype).
I have to say, I always found Bob's songs to be a bit boring and cheesy growing up. He would often sing the really really slow songs that I didn't like.
However, I admit that he is a talented vocalist. I also liked him fine on group songs, like "I've Got Two"
I'm also in agreement about Jerry Nelson. While it's not SS, his performances as Floyd (like "Gone Fishin'" with John Denver) are awesome. And the Count songs are fantastic, of course...
Sorry to keep going on, but let's not forget the awesomeness of Henson in Ernie's "I Don't Want to Live on the Moon." Rarely has a song been sung so sweetly and earnestly. JH may not have been the world's greatest vocalist, but his notes were dead-on and perfect for the characters he played. I mean, "Mahna Mahna?" "Meet Me At the Busstop?"
Sheer jazzy brilliance.
-David