Rand Paul B+
Pro: Like many likely candidates for the nomination, Rand is relatively young- he can also capitalize on the grassroots system his father Ron Paul created- which spans the nation and even gets fed by activists in other nations. His power to motivate independents could be severe. His popularity in Iowa and New Hampshire (both of which saw huge independent turnout for Ron Paul) would likely grab him the first two states.
Con: Rand has a tendency to misspeak and misattribute quotes while he speaks; neoconservatives, who form the fiscal structure of the mainstream GOP, will need to be convinced he won't attack their business backers.
Chris Christie B-
Pro: Christie is a relative moderate, who has the unique ability to steal votes in the blue northeast and midatlantic- he may, however, lose New Jersey, where he serves as governor.
Con: Christie is seen as a traitor by the tea party and his seemingly close relationship with Obama after the hurricane, plus the bridge scandal, has already tanked his popularity.
Ted Cruz B-
Pro: Cruz is hailed as a sort of hero by the tea party, and there's no question he'd have their support- of all possible candidates only Cruz can combine the tea party and the neoconservative base.
Con: Cruz tends to become aggressive and almost theatrical while speaking which could harm his debate skills. It's also not completely clear if he's eligible to serve as the president given his Canadian birth.
Newt Gingrich D
Pro: Gingrich is a perennial candidate- he did win several states in 2012, and enjoys heavy support in the deep south, where he could prevent Hillary, if she runs, from taking Georgia and Arkansas.
Con: Gingrich has never been taken seriously by the Democrats, and even in his own party, he is rapidly losing favor.
Rick Perry C+
Pro: He had an originally good showing in 2012, which only faded after he was clearly outcompeted in debates. He could grab all the money the neoconservatives can funnel to him through Karl Rove.
Con: His weak debating skills plus his home state being already red are not good signs for a Perry run.
Rick Santorum D+
Pro: Santorum came in second place in 2012- and the GOP has a habit of nominating runners-up. Santorum would have the evangelicals fire-eyed and at his feet in moments.
Con: Santorum is seen as extremist in his religious views- his presence would automatically energize whoever runs as his opponent with votes from women and minorities.
Jeb Bush C+
Pro: Jeb's time may have finally come; and as a Bush he enjoys massive financing potential, through which he can outspend opponents- and money above all else determines the GOP candidate.
Con: He's a Bush- and there are die hard Republicans who have sworn off Bush family members for life due to the last two that have served.
Marco Rubio C+
Pro: Rubio has the capability to break the hispanic vote more evenly- a huge advantage in contested New Mexico and Nevada.
Con: Rubio has been quiet lately, and is a distant candidate at this time- it's not certain he intends to run.
Hillary Clinton B
Pro: Hillary has the backing of Bill, a popular president, and Bill controls a massive number of older Democrats. Her speaking skills are considered above average, and she could destroy the GOP by taking both Arkansas and Georgia.
Con: Benghazi will hurt her credibility, and all the Bill era scandals will be back in full light as well if she runs- her closeness to Obama is a hindrance. It's not clear the younger Democrats want her to run, as they snubbed her for Obama despite her winning the nomination in the vote tally.
Bernie Sanders B
Pro: Sanders is an excellent speaker and enjoys high popularity in the Democratic party- that he would win New Hampshire (which neighbors Vermont) is all but a given.
Con: Sanders has been an independent candidate before- and sometimes is known to attack his own party (including Obama) at times, and is a self avowed socialist.
Dennis Kucinich D+
Pro: Kucinich is seen as a deeply antiwar candidate in an increasingly antiwar USA, and has some ties to libertarian issues.
Con: He is now aging, and is seen as a fringe candidate.
Howard Dean B-
Pro: Dean has a decent record and fairly high popularity in several swing states- his work as chairman of the DNC has surely earned him key allies.
Con: In 2000 he self sabotaged what was at the time a good campaign, and overnight went from a front runner to dropping out.
Elizabeth Warren B-
Pro: Warren is a popular Democratic senator, known for giving good speeches- her recent plan for student refinancing will win her the youth vote.
Con: Warren is seen as relatively far left and would likely not provide much of a challenge in midwestern swing states, which have seen a shift to the right in the last several years.
Joe Biden C+
Pro: Biden appears to be a sort of modernist version of Lyndon Johnson, believed widely to be a better tactician than Obama himself- his debate skills are good, despite his general verbal shortcomings.
Con: Biden is known for making frequent verbal mistakes and has been kept largely out of the limelight by Obama- possibly because of the same said verbal mistakes, and possibly because he's being groomed for a candidacy and "kept above the fray."
Al Gore C
Pro: Gore already came monumentally close to the presidency- he enjoys high support among the youth, and might be able to grab some of the right leaning inependents who remember how bad Bush was.
Con: Gore is seen as fairly far left and will do badly with moderates- his fixation on global warming did not endear him to some industrial unions.
John Kerry B-
Pro: Kerry did well in 2004 and still enjoys two key advantages; firstly, a massive fortune that allows him to finance his own campaign at will, and secondly, the remnants of Bush era antiwar sentiment. It is possible that, had the election been held a year later, Kerry would have won.
Con: Kerry is considered "boring" and was never known to make a particularly compelling, stellar speech. Although Kerry would enjoy high party support, his ability to sway independent voters can only be middling at best.