Thanks to Masterofsparks, I learned that Bob Seger and ZZ Top were once really good. So I'm preemptively stealing his likely suggestion.
I would've probably mentioned both of these. Also Rod Stewart and Elton John, who have already been mentioned.
As far as a big, obvious one, how about Aerosmith? First four albums are amazing, next few are not so hot, and starting with Permanent Vacation, everything else is pretty rotten.
As for a band that started not so good and got great, Thin Lizzy is a big one. Their first 3 albums are the sound of a band playing with different ideas, trying (and mostly failing) to find their sound. Once they hit "Whisky in the Jar," they found it and got great and stayed that way till the end.