There are a few obvious ones. The Destiny trilogy comes to mind. I would have loved to have seen the Romulan War on screen (though the books were written for that because I think they realized that was never going to happen).
But the one I'll throw out there is a favorite of mine that is probably a dark horse. "Chain of Attack" by Gene DeWeese is one of my favorite all time Star Trek novels and I can easily picture that on screen.
The basic gist of the story is the Enterprise passes through a portal to another galaxy. I forget exactly why, but they can't just go back through the portal to get home and I seem to remember there was no point hanging around the entry point. So they decide to do some exploring, partly, of course, to see if they can find a way home. There is a uneasy feeling as they pass planet after planet devestated and uninhabited due to apparent war hundreds of years before. Eventually they encounter two civilizations that are at war, and have been for generations. Captain Kirk attempts contact with the two and attempts to get the 2 sides to talk to each other (even though IIRC they have not spoken directly in ages). I won't say anymore to avoid spoiling the story but I really loved the exploration angle, they really were where no one has gone before. And it requires Kirk to draw on some of his diplomatic skills and just his ability to convince others to do things. And there's no way for the Enterprise to get help since it's in another galaxy. They really are the only ship in range.
