I've owned every Gallop Racer in the series for the PS2 and I have to agree with the earlier caller, Gallop Racer 2003: A New Breed is champion! I also have a copy of 2006 I'd be happy to give away.
Note: There is a horse racing nut out there that took all the fictional S and A graded horses and made a list of their real-life counterparts. It can easily be found on the Internet with a simple Google search.
Nevermind. I found it for you:
http://finalturn.com/database/gr2003.php