I was thinking about my most played track of each Britney album, and it happened I was quite surprised with some results...
According to my last.fm:
My most played track of both Circus and Blackout is "RADAR"

"Godsoline" for FF
"(I Got That) Boom Boom" for In The Zone (A SUPRISE RESULT)
"Don't Go Knockin' on My Door" for OIDIA
"Slave" for Britney and "BOMT" for BOMT.
Curiously, Toxic, for example, holds an accumulated number of plays much higher than Boom Boom, but because it's spread through different albums (her GHs), its total plays are shared, same goes for "Stronger" on OIDIA...
What's your chart?