This Rythem is a Dancer cover is better than the one which charted in recent months
https://www.youtube.com/watch?v=k7xtQicTqUk

This Tove Stryke Britney cover is pretty good and makes the bubblegum pop more grown up for a new era
https://www.youtube.com/watch?v=OY93Vwm19zI

If anyone mentions Ryan Adams version of Wonderwall i'm gonna scream. It's so overrated.

Can't think of any right now but I can certainly say what would not be a good cover and that's anything that takes an upbeat song or at least one with a little oomph in it and strip it bare with a (usually female) vocalist singing softly with plinky plonky music in the background (featured on adverts like John Lewis)

aurora did a good dance version of 'ordinary world' in y2k.

https://www.youtube.com/watch?v=FA39yqwnX98

Always liked this one:

https://www.youtube.com/watch?v=acULghgYUg0

absolutely agree.
There was this incredible dull singer songwriter cover of You're not alone by Olive which totally took all the energy out of the song, a couple of years ago.

I do like the mash up that Bastille did of The rhythm of the night, though i am not fond of Bastille

I'm struggling to thing of many mainstream 90's songs with covers that I like. Every time I seem to come up with something from the tail end of the 80's.

Coincidentally one I can think of is another cover of a song already posted "Bay One More Time". This time by Keisha Patterson, who's possibly better known as a backing singer than for her own releases:

https://www.youtube.com/watch?v=2OM3Jcp0klM

Sacrilege.