The 26-year-old Italian won the 2012 crown with DAMS and has graduated to a reserve Formula 1 role with Lotus this season.
He believes that Swiss driver Leimer, who is entering his fourth year in GP2, and drives for Racing Engineering, is the most likely driver to come out on top in the championship fight.
When asked for his championship prediction by AUTOSPORT, Valsecchi said: "If I have to make a list, it goes Fabio Leimer first, James Calado second and Marcus Ericsson third."
DAMS has won the past two drivers' titles, each time using a vastly experienced driver to do so. Romain Grosjean took the 2011 crown in his third season of GP2 while Valsecchi was in his fourth campaign last year.
This season the French team has recruited Marcus Ericsson, also in his fourth season of GP2, to head its line-up alongside Stephane Richelmi.
But Valsecchi does not believe that the Swede will give the champion team another title.
"I will always support DAMS because it's the team that gave me a chance to do something and because of how they helped me," he added. "But I don't think Ericsson is as strong as Grosjean or myself.
"That's just my opinion. I've spoken to my old engineers, who also worked with Grosjean, and they say that Ericsson is very strong and that he will fight for the championship. I still see him third though."