YouGov Poll -- taken between December 14th and 16th of 1,000 nationwide adults, with a 4.2 point margin of error. The chart above was the opinion only of the poll's 296 Republican respondents (so the margin of error would be a bit higher).
As you can see, Chris Christie leads the poll with 18% -- but that far from a number that would make him a real favorite. With no candidate getting even a fifth of the vote, the only real thing that can be said is that there is no favorite yet. This is still a wide open race that anyone (even a currently unknown candidate) could win.
It does look like Paul Ryan has taken a hit among Republicans because of his reaching a budget agreement with Democratic Senator Patty Murray. At the end of October, Ryan was the choice of about 22% of Republicans. He now is the choice of only about 10%.