My favorite? Reepicheep, of course!
Oh okay, I'll give a better answer than that. Of course I love Aslan, but as a kid, I didn't understand Aslan. It was weird: you got this powerful witch, a kingdom of talking animals who are so afraid of her they don't even mention her name, and one random lion stops her; even with the explanation of the Deep Magic, the whole thing about him coming back from the dead just seemed a bit "deuce ex maquina" (though I didn't know that expression back then). Then comes Voyage, when he says he's in our world too - which had me thinking how badly a talking lion would be treated in our world (thinking of circuses and stuff like that). Even after I came to Jesus, it was years before I ever learned what symbolism was or how people (and not just Lewis) use stories to communicate messages that are supposed to mean something to us beyond the story. Now going through the books again, I can't help but see it everywhere, and I'm loving that, but I can't really call Aslan my all-time favorite because, for most of my life as a Narnia fan, he wasn't.
But Reepicheep, OMGosh did I like him! Of course as a kid, I just thought he was awesome because he was not afraid to take on a dragon, even though he was just a mouse. He was a swashbuckling adventurer, possibly the smallest creature in Narnia but also one of the bravest. And now many years later, I love his character more than ever! He was a Narnian knight, loyal to King Caspian and with a deep desire to see Aslan's country. He's not without his faults, but IMO that just makes him a more "real" character. So I can't think of any other non-human character in Narnia that I'd rather call my favorite.
In fact, when I started coming around NarniaWeb last month, and saw all these very Narnian nicknames, I dropped my usual online nickname of the Geek on Skates, to officially join as Geekicheep! That's how much I loved that mouse.