Asimov is fantastic; probably my favorite writer after Clarke. I'd recommend all his stuff, but especially the Robot and Foundation combined series. The books that impressed me the least were the Galactic Empire novels. His writing style was deliberate. He felt that communicating complex ideas was best done by writing simply and clearly; not the only way to do it, certainly, but it worked for him. I also second the recommendation of Psychohistorical Crisis. It's Big SF like, say, Vinge's "Marooned In Realtime," and very enjoyable. However, I definitely don't recommend the Second Foundation Trilogy by the Killer Bs. It was well written (of course), but not good Foundation. It was inconsistent with the original and-- well, I won't post spoilers, but let's just say it introduced an idea which was a great SF concept, but completely inappropriate for the Foundation Universe.