I never did read any of the books by which he was best known (other than ST): The Moon is a Harsh Mistress, The Number of the Beast, Stranger in a Strange Land, etc. Partly because my liking of SciFi is decidedly mixed and partly because Job stopped my reading of him, but mostly because the blurbs for the books just didn't appeal to me.
There is no denying that RAH had a big impact on SF, but most of his stuff just wasn't for me.
Tuor
His stuff is definitely mixed in quality. Some is excellent, and others leave me saying "WTF?". I've actually read a great many of his books over the years, including most of his more famous works (and a number lesser ones).
My take: "The Moon Is A Harsh Mistress" was definitely a good read. That was one that I would recommend. "Stranger In A Strange Land" was okay, but not as good as you would think considering how famous it is. It had a decidedly 1960s feel to it, and comes across as dated in other ways as well. "Number Of The Beast" was quite mediocre. There are a bunch of other ones, some of which were decent stories, others which quite frankly sucked in my opinion.
However, the one book he wrote which I found to be thoroughly brilliant: "Time Enough For Love". It is a deep and complex book, but it is an intriguing read and a bit of a departure from some of his other stuff. It still has that quintessentially Heinleinian flavor to it, but the style is a bit different and the concept and content is much deeper than in most of his other books. It isn't for everyone, but I would recommend it as one of the very best books he ever wrote, and it would certainly be at the top of my personal list.