I think EVERY and EACH track of Circus could be the next single released...
anyway if I were Britney (*o*) I'd act like this...

1. Womanizer
2. Circus
3. Out From Under
4. If U Seek Amy
5. Kill The Lights
6. Radar (as Bonus Single)

However...
<3 GO*BRITNEY*GO <3
LUV YA 4 EVER!!!