While I've got no problem with the question being asked, it really is the majority of musicians who undergo a decline over the years. It's simple mathematics. You have your whole life to write your first album and just a couple of years to write the next. Generally, the material that makes up a band's or artist's first (or first few) albums has literally had more time to ferment and be perfected than the later stuff.
The only exception I can think of is Limp Bizkit, who wrote their first album in two weeks and it was the best thing they ever did (though that isn't saying much).
The more interesting question, and tougher to answer, would be if you can name any musicians out there who get better over time.
I could name a few, but that's not the drive of this thread, so for the sake of this post, I submit:
Arctic Monkeys (I saw someone post it up the page a little, but really, Favorite Worst Nightmare was one of my favorite albums of the decade, then they got boring), Rise Against (also previously mentioned), and The Offspring. I mentioned these bands specifically because I think they all suffer the same problem: that they've simply slowed down. Sure, you can make arguments for all different facets of songwriting in which they've begun to lack, but the high energy is what made these bands special in the first place. It was the speed and enthusiasm they displayed in the early years that drew fans, and for whatever reason (most likely age), their tempos have been dropping, and along with that, my interest.