OOTSP is the easiest of the series, in my opinion, to pick up and just enjoy. It's a joy from start to finish.
Perelandra...well, I always enjoy it, but the parts at the end ('rotating around the field of Arbol') completely lose me every time. I've read the book twice and still don't get it.
THS...is a good book, and would be my favorite of the three if it wasn't just so darn
long. It seems to take
forever until things finally start turning around for the better. To me, it wasn't as depressing as
The Last Battle, which I read a few months ago for the first time, but still...I'm not sure that I'll be listening to (the audiobook version of) THS again any time soon.