I can't think of that many and I listen to a lot of different artists, btw. I think it's possible to have a consistent or increasing amount of quality on albums for a while but eventually... one hits a bump in the road, esp. if they have a lot of albums.
The only one I can think of so far is Fiona Apple. She's had 4 albums so far so it's a little easier since she hasn't hit a rough patch yet and she really evolves with each release. My other faves either have too few albums to make a definite call or too many and have more than one.
Britney up until Blackout got better with each new album.