Personal preference aside..
Dems:
Clinton
Patrick/Booker (one or the other trying to carry Obama fresh/modern "change" mantle)
Cuomo
Howard Dean (better chance of him being the liberal candidate instead of Feingold)
Biden (would throw hat in and probably bomb out)
also rans would be maybe a southern governor like Beebe, Klobuchar or Gillibrand, possibly a Hispanic candidate like Luis Gutierrez or Villagarosa, O'Malley would bomb out like Tim Pawlenty..
GOP:
Nikki Haley (suprising, but she has a good record, common sense, charisma, female and minority, southern base and tea party likes her)
Chris Christie (if he loses NJ gov race to Booker, he would run, if he wins, I think he doesn't run)
Rand Paul (Paulites will transfer their support to him)
Paul Ryan (next in line kind of thing, still young and somewhat popular)
Scott Walker (firebrand, popular with tea party, well connected and won twice in a blue state)
also rans: Jeb Bush, cause of Bush fatigue, maybe Jon Huntsman runs again, Martinez I dunno, possibly some tea party firebrand type.. like Allen West or something. Rick Perry could run again too, but given how bad he did in 12, don't give him much chance. Santorum could run again, but he probably will get pushed out and won't have the same evangelical/conservative support. I could see Huckabee running again too as a "happy warrior" evangelical type, but might be a bit too much on social issues.
As for Rubio, time will tell, he might still be a bit too young, and may or may not have some skeletons in closet issues.
At this point, who do you think will be the top 5 candidates in each of the two major parties' primaries in 2016?
In no particular order, I'm thinking:
Democrats:
Hillary Clinton
Andrew Cuomo
Russ Feingold
Martin O'Malley
Amy Klobuchar
Republicans:
Jeb Bush
Chris Christie
Susana Martinez
Rick Santorum
John Thune
Feel free to speculate. 
