The Horse and His Boy isn't one of my favourite books of the series, now. I think when I was younger it might have been one of my favourites -- I think I owned a BBC dramatisation of it, or something like that, which was pretty good and helped my very positive memories of it. Shasta wasn't my favourite protagonist even then, I think, though I did love Aravis and wished we saw more of her.
Looking at it now, The Horse and His Boy feels like a bit of an aside, really. Shasta/Cor, Aravis and the two horses aren't that important, ultimately, to the history of Narnia. The other books all cover pretty important moments -- creation, freedom from oppressive regimes, new rulers, daring rescues... This book hardly has Narnian characters in it, and when it does, they're not absolutely central to the plot. It's more about Archenland and Calormen -- which is nice, in seeing more of the world, but it just doesn't seem to quite fit.
The writing is still beautiful, the narrator still a benevolent omniscient storyteller type. The book makes me want to go to Narnia as much as the others do. If I say I don't love it as much as the others, that doesn't mean I don't love it very much.
But now for Prince Caspian!