Which book or series, IYO, was his best?
The Norby the Robot series. Its what got me hooked on Asimov as a child.
I really loved The Caves of Steel and The Naked Sun. Next I would place the original Foundation Triology.
Slight OT comment, I only just found out that Asimov died of AIDS, acquired from blood products during heart surgery. I was floored! But now I REALLY understand his last editorial in Asimov’s.
Another vote here for The Caves of Steel.
bayonet1976, Where did you hear that Asimov died of AIDS? I read that his body had been damaged somehow by the heart-lung machine used during his surgery, but nothing about AIDS.
I vote for Caves of Steel too. Also his autobiography (3 books!) is fun to read.
I just searched and found the book his wife published (It’s Been a Good Life), that revealed he died of AIDS. Have to get that one.
For me, I, Robot is his best, followed very closely by The Foundation Trilogy. Fun, intriguing and worth reading.
I actually liked The Gods Themselves mostly for the sex scenes. 
However a better choice would be The Caves of Steel or Foundation and, of course, “Nightfall” and “The Last Question.” Finally, his story “The Feeling of Power” was his most lasting contribution – giving the world the term “pocket calculator.”
And, yes, it was revealed last year that he had died of AIDS from a transfusion. The doctors insisted he not tell anyone (covering their asses more than anything else) and, despite the urging of his wife and others, he did as the doctors asked.
The Foundation Trilogy for his longer stuff, and “NightFall” and “The Last Question” for his shorter stuff.
I haven’t read all that much of his material, but my favorite would have to be Nightfall, followed by Foundation and Empire and Second Foundation. In general, I find that his books have lots of creativity and good plotting, but rather poor writing.
While I am agreeing on all counts with ITR, I found “The Gods Themselves” to be his best written novel. It won the Nebula award in 1972, which is no small achievement.
OK, they’re not as important as any of the above, but I’ve always had a fondness for his “Wendell Urth” series of SF mysteries. It’s a tricky genre to write, and he handled it well. Besides, Urth manages to be another homage to Nero Wolfe in his reclusiveness and a reminder of Asimov’s reluctance to fly.
I have to say the Foundation trilogy. Thart’s the trilogy – in Asimov’s memory we should just ignore the awful sequels, starting with Foundation’s Edge.
Another vote for Caves of Steel, and I just got through rereading The Naked Sun. Great book. From what I’ve heard, R. Daneel is a very popular character, but I always liked Lije the best.
I, Robot was spectacular, and still is.
As to the AIDS question, in the new biography, It’s Been A Good Life, his wife writes that he got HIV from a transfusion from his bypass surgery in 1983 when not a lot was known. He died of AIDS complications including kidney failure.
I was always more stuck on his non-fiction work. The man could write about anything!
He had some very entertaining short stories. I love the ones where he 'll write a 20 page story all to justify a one sentence pun.
Also, The Sensuous Dirty Old Man is the best title for anything, ever. Too bad the book itself isn’t very good.
Ah, the best story he wrote was “Breeds There A Man?”. It has a great plot, it’s well written and it ends in a shocking way.
Slee
My personal favorite of his novels is Pebble in the Sky. He classes it as a “Galactic Empire novel”, along with The Currents of Space and The Stars like Dust (both also pretty good), but there’s no strong connection between them.
sleestak, refresh my memory: Which one was “Breeds There a Man”?
Well, as a lifelong mystery fan, I’ve always been partial to the Black Widower and Union Club stories. Not that I could always solve 'em, but I never could solve 'em all.