You know, I think this is something that, for me, has changed (and continues to change) over time.
When my 3rd grade teacher first read it to the class way back (what got me into the Chronicles of Narnia), Aslan was my favorite simply because lions were my favorite animal, lol -- I was obsessed, had plushies and statues everywhere of lions, every time I was at the zoo, they were the first animals I wanted to see, and I legitimately used to ask my Mom on a regular basis why I couldn't have a pet lion.
(At that age, any religious significance went right over my head -- I didn't learn until years later that he was meant to be Jesus Christ -- so as silly as my "oooh, a lion!!" reasoning may sound now, you must realize this was coming from an eight-year-old kid, lol.)
And Aslan was so full of warmth and kindness on top of being a lion, he was immediately my favorite, and I remember trying so hard not to cry in the middle of class when our teacher read us the part where Jadis kills him.
Then the movie came out (and I didn't see it until about a year after the fact) and I was closer to my preteens and more boy-crazy than ever (that's a lie, I'm an adult now and I'm still constantly crushing on celebrities, but whatever, lol), so of course, for a while Peter was my favorite because I thought William Moseley was really cute.
Then I went through my Edmund phase because I suddenly thought Skandar Keynes was cute (even though I still cannot for the life of me pronounce his name)...Not sure I'd call him a favorite anymore, but for sure, the older I get, the more I can appreciate his character development throughout the series.
But now, at age 22, I have to say, as cliche as this answer may be, my favorite is probably actually Lucy. She immediately let Narnia into her heart and had the wonder and strong faith to believe in Narnia and in Aslan all along...I feel that some may overlook her strength, actually. It may not be physical strength, but emotional strength, and/or strength of faith, is nothing to sneeze at.