4 is Beyoncé's best album to date. B'Day and I Am...Sasha Fierce, in my opinion, are both equally good and come right after 4. I've never been able to pick between the two. They're masterpieces
There is some filler on B'Day... but the tracks that stand out stand the **** out. Upgrade U, Irreplaceable, Check On It, and Ring The Alarm are incredible tracks.
There is also some filler on 4... but the songs have more quality in them.
It's really hard for me to choose, but I'll pick 4 only because some tracks on B'Day haven't grown on me even though I've listened to the album a good 10 times now (haven't gotten into it until recently).