In the latest head-to-head matchups, Obama does notably better against McCain than Clinton does. Both Democrats beat the snot out of Romney.
For the record, I'm not sure how reliable these polls are. Hillary, my colleagues keep reminding me, is pre-slimed. Everyone in the country knows her negatives, and they're not likely to get much higher. Obama, meanwhile, has enjoyed a very nice media ride over the past few months (hell, years). Should he become the nominee, his past (his years doing crack in a madrassa with Saddam Hussein, etc.) will come to the fore, and he won't look quite so pristine. Of course, the same lesson goes for that other media darling, John McCain.