It's way too soon to speculate as not everyone who plans to run has announced yet. It's a grueling process going through the debates, doing countless interviews revealing one's stance on issues all while avoiding making any idiotic statements.
As of now it's probably Jeb Bush and Scott Walker who have the best chance of winning the Republican nomination.
A lot can happen though between now and then.
Back in 2007 when people were looking at Democrat candidates for 2008 Hilary Clinton was the heavy favorite to win the nomination. After Iowa Obama pretty much ran away with the nomination.