I sometimes find that slowed versions (Daycore are slowed down versions of songs while Nightcore is speed up) of certain songs sound better or just as good. I personally like the slower version more than the original Buble one. Out of all the version I heard which includes Nina Simone's rendition I like the Michael's the best. Out of curiosity which rendition of Feeling Good is your personal favorite?
I agree as in general it's really up to the final result of the cover in the end. Sometime slow is better, sometime faster is better or sometime totally different kind of style (like brass, ska, metal ...). In this specific case, I tend to go roots as to me there's no way to beat Nina Simone's except by herself (live at Montreux is quite an experience). When it comes to classic, there's another song that can be reworked infinitely and it's Fly me to the moon. The most unexpected and astonishing cover that come to my mind is Slayer's Raining Blood by Tori Amos. You came from a speed metal version to slow, dark and gritty one. I really love it.
lol If you weren't a few hours ahead of me!!! Saw on my phone when my alarm went off and didn't hit the snooze! The only reason to get up on Mondays really. :)