Personally think Hillary did a very good job as Secretary of State apart from Benghazi, which I refuse to blame solely on her.
She really didn't do anything as Secretary of State. She was nothing but a mere figurehead mostly due to the extreme micromanagement of Obama's inner circle on foreign/national security policy. She was shut down many times by Obama and she left the State Department rather frustrated like many important Obama cabinet members (like Robert Gates, Leon Panetta, and Chuck Hagel).
Think she has the most experience and also ability to be president. There is no better Democratic nominee. Everyone else mentioned in this thread is either too inexperienced, radical or simply useless. Biden has been nothing more than an advisor in his role as vice-president and he's never going to win a race going by his debates in the last election cycle.
If you think that Biden was just a mere adviser to Obama as Vice President, you clearly don't know how Biden operated. Without Biden, literally nothing would have gotten done. For as much blame as the Republicans get for their inability to work with Obama (deservedly so), Obama also deserves a lot of blame for his inability to reach out to Congress. Without Biden to make deals with McConnell and Boehner, the United States would have gone over the debt ceiling, the shutdown would have lasted much longer, etc. Biden has been nothing but an integral and extremely loyal member of the Obama Administration.
Warren is too inexperienced and radical for her to be even considered.
But Warren represents a direction that many Democrats want to go. What we're seeing is a battle for the soul of the Democratic Party. A battle between the Populist Progressives (Warren, de Blasio) and the Traditional Progressives (Clinton).
The emails will hurt her. They won't end her (potential) campaign, but it gives the Republicans ammunition. Maybe it's because I like her but I don't find serious problem in her using a private email server, unless there is a smoking gun on Benghazi of something.
I don't see the e-mails as a whole as a problem for Hillary's campaign, but they represent a fundamental flaw with Clintonland and in today's day and age, I think people are going to get fed up if Hillary does not change her ways.
At the moment she's the Democrat's only shot. And considering the Republicans potentially running.. Cruz, Paul, Christie and Bush.... America would be very very lucky to have Clinton in the White House.
I would say that candidates like Joe Biden, Jim Webb, and Brian Schweitzer would be good nominees.