And I was really into all of Britney's albums lately and I think that "Britney" and "In The Zone" are her best albums.

Not a single bad song.
"...Baby One More Time" and "Oops!...I Did It Again" are great too (I prefer "Oops!" out of the two), but they are more teen-pop influenced, so I can't really compare them.
"Blackout" is great too, and I love Britney to death, but I think it's slightly overrated. :/
"Circus" is probably her weakest album to me. :/ Maybe cause it was too safe. :/