This thread makes no sense to me. If an artist can take an existing song and spin it in a way that makes it interesting or unique from the original, awesome. I love hearing more music, and I love hearing unique perspectives on existing songs. The level of variety that can be achieved with even small, subtle changes to a song is staggering to me, and I love hearing it.
If someone says that the cover is better, then to them it is. Having your name in the writing credit of a song doesn't give bonus points in terms of the quality of the song - By that logic the demo tape of a song is the 'best' version, because it was the original recording; the album version was corrupted by the producer and sound engineer and all those other assholes who put their own touches on it. If you want to argue that there's some objective 'best' version that's separate from your own opinion, go right ahead, but it's a completely ridiculous (and pointless) argument to make.