All of her albums are amazing and all of them have great music. It's incredibly hard to pick the best one but I'd say 'Circus' and 'Femme Fatale' are the two best albums.
I like Femme Fatale but I also like the PEROGATIVE one a lot zaidi because I like how she used to sing about love....not how she sings about how she has sex cravings now!
It depends on my mood: sometimes my favourite Britney album is In the Zone; sometimes it's Britney Jeans; and sometimes My Prerogative: Greatest Hits (if including greatest hits). At the moment, though, my favourite is Femme Fatale; I upendo the almost every track, but my favourites are Inside Out, Hold It Against Me, Till the World Ends, Trouble For Me, and I Wanna Go.