This really a double-pronged question...who earns the most, and who saved the most. There could be some girls making $100,000 p.a., but only having $10,000 left at the end of the year versus girls who make $50,000 but are able to save $25,000, for example.
If we're talking mainstream porn, then my top 5 guesses (in no particular order) would be:
Stormy Daniels (Wicked contract, feature dancer, occasional mainstream cameos, has Stormy Entertainment deal w/Wicked, plus writes and directs as well as performing, etc.)
Tera Patrick (Still popular, plus owns her Teravision output)
Belladonna (I know Jeff hates her, but she's wildly popular, owns back catalog via Evil Angel. Mainstream exposure has hardly hurt her either, paradoxically)
Sunny Leone (Vivid contract girl, great fanbase due to her ethnicity making her very unique, now has her own distro deal w/Vivid too...has always struck me as one of the smarter, more business-oriented girls out there)
Hillary Scott (Million dollar SexZ deal, allegedly...)
Jenna Haze is probably knocking on the door too, or somewhere thereabouts.
(I left out all the really obvious/ubiquitous stuff like website, signature toy line etc.)
...but we shouldn't forget that there are a lot of web-exclusive girls or girls on the fetish side of things that are also pulling down some serious $$$'s too.