It wouldn't let me vote for some reason. Hm.
But, anyway. I was looking at the list and knew right away which was my favorite, and still couldn't feel justified in ruling the others out so quickly. So a list (like others I've seen here) seems to be in order (heh, no pun intended):
1. The Voyage of the Dawn Treader - From when I was smallest, ships have fascinated me. I'm one of those hopeless, Irish sort of romantics who like long songs about the danger of the sea and then still love the open water anyway. Aside from capturing the seafaring adventure of exploration, danger and the joy of the discovery, Eustace's own journey from a bratty kid to a redeemed young man is, to me, one of the most poignant, meaningful allegorical illustrations of the journey of a sinful human to a saved child. I agree with you on that one wholeheartedly, Crimson.
2. The Last Battle - A book I'd mostly read, reread and forgotten until sometime last year, while listening to the audiobook with my family. It very nearly blew my mind. The incredible grasp of culture as a whole is still staggering. The simple way in which Lewis weaves the story, with the masks, the lies, the self-deception afterwards, the lingering hope and raging, consuming redemption, calling and, adversely, the separation, still speak to me deeply. It calls to mind both Schaeffer's "How Shall We Then Live?" and another Lewis favorite, "The Great Divorce." This is usually the book I recommend most strongly to college students who ask about the series, though I do recommend reading "up" to it first.
3. The Silver Chair - I see it's unfortunately not as popular here. Still, one of my favorites and probably for silly reasons. I love the hugeness of their problems and the struggle the characters go through to "remember" what they've been told, however simple. Reminiscent (to me) of some of the letters in Revelation, now that I think about it.
4. The Horse and His Boy - Horses! Foreign lands! Oranges! Sand dunes! Deserts! Lions! Honestly. Few books are as fun as this one, non-Narnia included.
5. The Lion, the Witch, and the Wardrobe - I'm actually sad that this is near the end of my list. I think it might mean more to me, so I'm going to have to claim the last four (from THAHB down) are really ties. One of my earliest memories is of watching the old animated version of this, and the whole piece still inspires and moves me.
6. The Magician's Nephew - I love creation stories, and still giggle whenever I think about the animals planting Uncle Andrew. This still captivates me, from the cringing realization of Digory's mistake to the moment he brings an apple home and plants it. This is the sort of book I could write a book about.
7. Prince Caspian - I'm not really sure why, but if I'm trying to list them off the top of my head, this is the one I most readily forget. It was just never as fun or touching for me, even though it definitely has its moments.
So. Lots of words. A favorite series, though, and worth all of them.