Obviously Gaga...
Maybe Ke$ha come back stronger single-wise, since Gaga has NEVER been a single's artist and Ke$ha's lyrics are not so "in-your-face" like Gaga's. In other words, Ke$ha's lyrics are MUCH more radio friendlier (I love her lyrics

) .In terms of EVERYTHING else, like pop culture impact, album sales, etc, Gaga has always had the upper hand. It's like when Tik Tok blocked Bad Romance from #1 and yet Bad Romance impact was a lot more massive.
That being said, I'm desperate for Ke$ha music and I hope she smashes hard.