Rammussen polls makes the case that Edwards is the strongest candidate, Clinton the weakest.
1) McCain is by far the GOP’s strongest candidate in the general election. (Rudy would be stronger, but he’s not going to make it past South Carolina, much less to the nomination.) He trounces Hillary 49% to 38%, and beats Obama 46% to 43%. In fact, the only Democrat who beats him is none other than John Edwards, who decisively beats McCain 46% to 39%.

2) John Edwards is by far the Democrats’ strongest candidate in the general election. He is the only one who is not beaten outright by any Republican candidate: He ties with Giuliani at 44% each, but easily beats McCain, Thompson, Romney, and Huckabee, the latter two by double-digit margins.
3) Hillary Clinton is the weakest Democratic candidate in the general election. She loses to every single Republican except Fred Thompson and Ron Paul, and neither of those two gents is going to get the Republican nomination for president.
4) Barack Obama is in between Edwards and Hillary Clinton in strength in the general election. He does much better against Rudy Giuliani than either Edwards or Clinton, beaing Giuliani 37% to 47%, yet against every other Republican, especially McCain and Romney, Edwards is the Democrat that does the best.