I believe I have, at one point or another, read almost all of Crichton's (fiction) novels. Most of them, certainly.
Like many here, I really enjoyed Jurassic Park, Eaters of the Dead, and The Great Train Robbery. I thought Congo was good, too, as well as Timeline and Prey. I found State of Fear to be a bit of a drag, and I really didn't get the point of Next. I have yet to Pirate Latitudes (published posthumously).
Out of all of them, however, I though Sphere was his best. A very entertaining read, interesting premise, and a terrific ending. The movie was horrible, just horrible, and I would strongly recommend anyone who has not read the book to not judge it by the Hollywood disaster.