Well, here are a few:
'Big Hoops' by Nelly Furtado was a big risk, especially when you're making a comeback.
'Beat Of My Drum' by Nicola Roberts is kind of a risk as well. She had a lot of eyes on her and could've easily gone with a generic single - thank God, she didn't do that.
'Acapella' by Kelis - I mean, so gorgeous but a risk.
'Dirty' by Christina Aguilera was a big risk, considering she completely switched her entire image (look+sound)
'Overpowered' by Roisin Murphy was a big risk as well, considering the heavy image + electro pop.
'Every Single Night' by Fiona Apple, even though it's flawless + has a flawless video and it's a good representation of the album. It's still a really BIG risk to put something out like that today.
'Walking On Air' by Kerli because there was nothing like it on the radio, I think?
'Bad Girls' by M.I.A. - then again, it is M.I.A. so I don't know if this can be considered a risk