Moon Is A Harsh Mistress will always be his best book with Starship Troopers being a close second. I did like Number of The Beast quite a bit also.
I first read Heinlein as a teen back in the sixties. I've read everything he has written (many of them more than once) and just recently repurchased his entire collection and I am rereading them once more. It is amazing what I catch in my fifties that I would never have understood when I was 15, 25, or 35. The man was a master at multiple levels. You could teach a college level sociology course with just starship troopers as the text.