I see Madonna getting mentioned a lot here. Personally, I'd say her best album was Erotica, which came out in the early 90's IIRC, though to be fair she has made some great albums (and also some bad ones ) since then, with Hard Candy being the worst IMO.
The good thing about Madonna is that she will sample current trends but create something quite idiosyncratic with it. Looking back at her singles, they kind of sound products of their time but equally and more importantly, they sound quite unique to the music that was being produced around that era- a song like Erotica for instance is a totally unique song, unlike anything else that was being produced around that time.
The trouble with Hard Candy is that it didn't 'sample' from it's genre but parodied it - this is probably due to Madonna (unconventionally) handing over a certain amount of creative control to other artists - it didn't work and out of all her work HC will probably become to sound the most dated.
No doubt she will come back with something better next time around...the biggest problem for me now is her image.
Hard Candy proved something of a double whammy, not only was the music substandard IMO but we also had all the incidents with the adoption, divorce and her new toy boy lover. This was also another new low for Madonna - where previously she could control the media by taking those negative portrayals and turn them around to her advantage, creating her own media event that negated the negatives - this time around, there was nothing, what we were left with was a rather nasty image of her.
Have to admit, my opinion of her dropped dramatically, to the point where my appreciation for her work has been effected. This has also happened for me with REM. As soon as Stipe decided to become the new Mrs. Bono then my appreciation of them dropped and not just their new material but I can barley listen to their old stuff any longer. So I guess for me, the image of an artist is perhaps the biggest and fastest way of me losing interest.