From what I've seen, if I had a vote (which I unfortunately do not) I would vote for Sam Yoon. He convinced me when he said "an urban area must always grow, or it will die." He seems like he will be able to strike a good balance between encouraging downtown development, while handling the more far off neighborhoods.
Menino- we just get four more years of the same. Over his tenure, I must say I like some of what he has done, and don't like other things. He has made strides in some areas (i.e. Bike lanes), but he has been around simply to long, its stale. I feel like he gets a lot of votes now, because people are just not given an alternative they feel passionate about.
Flaherty- He seems so boring, and I think that will translate into the city. He means well, but his points and vision make him more suited to be the mayor of Plymouth than Boston.
McCrea- He is just a good foil in the debates that represents a lot of people's frustrations and unproven thoughts on CityHall and can say the crazy things Yoon and Flaherty can't. The votes for McCrea will be anti-Menino votes, and everyone of them will vote for the challenger in the final election.