If you're interested in comparing the number of false stories each side makes up about the other, one good place to look is Snopes.com. Check the page for each person and see how many negative hoaxes have been created.
Romney: 8 (I'm not counting the mixed-truth ones, or the one that said Oprah endorsed Romney, since that wasn't really negative toward Romney.)
http://www.snopes.com/politics/romney/romney.asp
GWB: 16 (Again, I'm not counting the mixed-truth ones. I'm also not counting a couple false ones that are not critical of Bush -- for example the false claim that Bush paid for the funeral of a young neighbor boy.)
http://www.snopes.com/politics/bush/bush.asp
Obama: 83 (Again, I'm not counting the many mixed-truth ones, even though most of those are critical of Obama, and are highly dubious. I'm also not counting several false rumors that are not particularly critical of Obama.)
http://www.snopes.com/politics/obama/obama.asp
As polarizing as GWB was, over 8 years in office, liberals created only one-fifth as many false rumors as Conservatives have created about Obama in just the past 4 years (16 vs. 83). Seems to me the Conservatives are much more creative and prolific in their false attacks!