According to a recent Survey USA poll,
Gephardt performs best against Bush among Iowa registered voters, losing by 7%. Following Gephardt is Edwards, who loses by 11%, then Kerry and Dean, who both lose by 12%.
These results would appear to contradict those of other Survey USA polls (PDF), the most recent of which show Dean to be the favorite, followed by Gephardt, Kerry, and Edwards. The difference: the sample. Democrats may prefer Dean, but overall, Gephardt is the best bet against Bush - at least for now.