In theory, my favorite would probably be the gallic swordsman - however, in practice, every single time I've played as the celts I've started with no attainable iron. I end up saving a warrior to later upgrade to one when I eventually want my GA.
In practice, the Mounted Warrior has, by far, been the most crucial. Sucking up that early golden age may hurt, but mounted warriors can seriously take it to spearmen. Take a little care to keep from exposing them and those crucial early control battles are cake.
I wouldn't call samurai a defensive uu. Sure, they have their defense augmented, but they are still an offensive unit. The most you could say about their defensive abilities is that they are somewhat 'dual purpose'. The extra defense lends to their offensive capabilities as you don't have to send a bunch of slowmoving pikes around to protect them. They make fantastic garrison units, but I find that has much more to do with their offensive nature, being able to strike out at the attacking force the turn before they rush the city.
A pre c3c musketeer would still be called a defensive UU, even with it's augmented attack... no?