Oh geez, I want all and every breed!
But if I HAD to narrow it down:
An Irish Sport Horse, to see their "fifth leg" in action
An Arabian, just because I've always wanted one
A Mustang, to witness the transformation from feral to trusting of humans
And an OTTB, just because I find them so dang cute with their gangly legs and huge ears!
But like I said, I would take almost any breed or cross any day of the week!