Gintama is the longest series I ever watched. I love it, including the background music. They have fitting themes for every situation.
After 150 episodes, however, I can recognise each piece after two notes, and can't help thinking: not again. It's costly, but new compositions would have been in order. Will wait.
The opening/ending on the other hand changes regularly. A hitlist:
All serious groups whose other works aren't bad either. With the possible exception of plingmin ;)