I love Shattered Glass, Out From Under, Blur and Unusual You.
Mannequin is awesome, too, (as is the rest of the album, naturally) but I think the next single should be slower and calmer.
We've had Womanizer, Circus and now If U Seek Amy, which are all fiesty and dancy.
Radar was one of my favourites off Blackout, but it just doesn't go with the tracks on Circus - you can hear that it was meant for a different album, so I don't think that should be a single.

However, I strongly believe that the next single Brit releases should be the one SHE wants to release. Afterall, she's the one who has to go around promoting it. Yes, it helps if it's a fan-favourite, but the most important thing is that she enjoys selling it, performing it and creating a video for it.

xx