2. HHB
Here's where things get tough...
3. SC
4. LB
5. MN
Those three are really just about tied, in my mind.
6. LWW
7. PC
I noticed that my top three involve cross-continental travel and exploration.
That sort of thing has always attracted me. Different cultures and environments, etc. VDT's theme of high adventure on the sunny seas is particularly appealing. No world-changing battles to mar the horizon!
I'm not sure why LWW is not as high up on my list as it is on most people's. The feeling of it just is not as appealing to me as most of the others. It's rather cold and gray for much of the book, and the ending is always kind of depressing; when they have to leave Narnia for humdrum England. Of course, most of the books end like that, but going from grown Kings and Queens to schoolchildren is a particular jolt!
Got to get off for the moment. I'll try and pop in later and finish up my analizations!

EDIT: I have.