For 3 magical beast HD, hippogriffs get some pretty good physical ability boosts, though it comes at the cost of severely reduced mental stats. Their primary weapons are two pretty weak claws, their secondary a bite that'll probably be used for weapon-wielding.
I keep mentally comparing this thing to the griffon, and not without good reason. The griffon (which got +0) may have the same physical scores and size, better natural attacks and armor, Pounce, and Rake, but all this is at four more HD than the hippogriff and we all know how easy Pounce is to get.
One could argue for +0 on grounds of the hippo's lack of thumbs and relative stupidity. I personally still lean towards +1, though. Hivemind, please opine.
Next up are hobgoblins!