Personally, I've always had a soft spot for the Foundation series. They were my first real foray into non-Trek SF and pretty much have given me a life-long interest in the idea of whether something like psychohistory/sociology would ever be possible or effective. I didn't even really mind too much that Asimov later appended the Robot series into the continuity, as it does explain the rather significant lack of any kind of artificial intelligence in the Foundation universe, as well as how a 20,000 year old empire could avoid the effects of Transhumanism, since the robot overseers would be loath to take such a step themselves, due to the extremely conservative programming they're beholden to, preferring instead to try to keep humanity stagnant, leading to the events of 'Edge' and 'Earth'.
I didn't even really mind the Killer B prequels. Taken apart, they're interesting thought experiments of the logical ends of some of Asimov's ideas. 'Fear' being most concerned about Psychohistory itself, 'Chaos' concerns itself with the Three Laws themselves and whether they've served their purpose in protecting humanity or if Asimov's 'Frankenstein' scenario came to pass after all. 'Triumph' seems most concerned with trying to come up with an explanation for the events of 'Edge' and 'Earth', as well as generally tie up continuity issues throughout the series. Of course, your mileage will vary as to how one likes any or all of those things, but I find it interesting to see Asimov's ideas thought through from a more modern, harder scientific perspective. I will say, however, that I didn't care much for the 'sims' or the 'memes', as they seemed to clash too much with the 'feel' of Asimov's universe. I have mixed feelings on the whole 'Robot religion' thing, too. While I can certainly see the merit in showing the conflict between the Calvinian and Giskardian interpretations of the Three Laws doctrine. I just think the metaphor was taken a bit far at times.
All that being said, I think the original trilogy is my favorite, as many others have said. It does an excellent job of showing the seeming inevitability of historical events, while at the same time, demonstrating that black swans like The Mule can always come along to change the whole game at completely unexpected moments, which is what makes history a fascinating study (too bad the classes are so boring... lol).