Other than special finish, I don't know if there are really any rare HTF Jr cars. I think the hardest to find car of Jrs is his 2009 GoDaddy NNS car. At least, I found it to be one of the hardest to find
His 2011 Paint the 88 car has a production of just 1297. In standard finish. Quite low for a Junior piece in standard finish. I don't know the production number, but I've found the 2010 Hallowdega car to be hard to find in standard finish too.