the whole album is amazing. its so hard to pick a favorite song!
my favorites are in this order:
Circus
Kill the Lights
Mmm Papi
Shattered Glass
Womanizer
Radar
If You Seek Amy (Hehe)
Lace and Leather
Mannequin
Unusual You
Out From Under
Phonography
Blur
My Baby

all in all the whole album is great.
but then i'd have to argue that all of britneys albums have been great!
i seriously love this album!!! dont think ill EVER get tired of listening to it !

and the video for circus is amazing!