Wow, tough call. If I was playing a guardian fighter, I'd probably pick Dragonborn. Breath marking is just about priceless. I like Goliaths but I'd rather be battlerager than guardian with a Goliath. Dwarves would be my other potential choice, but they are too good at being tempests or brash striking greatweapon fighters. But their defensive abilities are unmatched. Can't be pushed, hard to knock down, great opportunity and combat challenge attacks, second wind as a minor, and dwarven durability make them hard to beat. Even their paragon paths work well for a guardian fighter. I'm voting dwarf.