[Foxbat]

Personally, my favourite is Stranger In A Strange Land (and that may be a biased opinion - let me explain why):

Stranger was the first Heinlein novel I read - and I enjoyed it immensely. Then I went on to others (I Will Fear No Evil, The Number Of The Beast, Job, etc.) and I noticed a disturbing pattern. It seemed to me that all the main characters in each book were merely transplants of the last. It felt that, despite the name changes, I was still reading about the same people with the same traits and similar resources (they all tend to be reasonably attractive, sexually uninhibited and financially sound enough to go on whatever advunture was required of them).

I stopped reading Heinlein because every book just seemed to blend into the next until the whole body of his work became a massive blur. Hence my choice (and probably because it was the first, it is my favourite).