Star Trek-The Motion Picture: Supposedly Roddenberry himself penned this. It is quite interesting. Some interesting information on human life in the 23rd century that doesn't really match up with what is established later. (New Humans, emergency starfleet implants, etc.)
Star Trek II: The Wrath of Khan: This was actually much darker and more graphic than the movie. Very detailed descriptions of Khan torturing the Regula I scientists. Good characterizations of Saavik, Peter Preston, etc.
Star Trek III: The Search for Spock This is a VERY interesting novelization. It's almost completely different than the film. Certainly, the general plot is the same, but there is so much difference it's almost unrecognizable. Nearly 3/4 of the book takes place before the Enterprise is stolen. Tons of stuff with the crew before they get to spacedock, and then when they are on Earth.
Star Trek IV: The Voyage Home Haven't read it
Star Trek V: The Final Frontier Strangely enough, I find this to be the most entertaining of all the novelizations. There is some good background to the Vulcan culture, and Sybok specifically. There are some good details about the visions Sybok has seen, as well as some very detailed explanations of some of the other character's secret pains. Most interestingly, there is a reasonable explanation as to how the Enterprise and the Bird of Prey get through the Barrier unharmed.
Star Trek VI: The Undiscovered Country Another excellent novel. I think JM Dillard is a very good writer. The best contribution in this novelization is the addition of a background story about raids that have been taking place on Federation colonies near the Klingon boarder where lots of innocents have been killed. It gives a good explanation as to why everyone is so tense about Klingons.
Star Trek Generations The best parts here are some added parts about Sulu and the Excelsior crew reacting to the apparent death of James Kirk on the Enterprise B.