D.C. Fontana. Not only did she write some of the best episodes of the original series and animated, but Vulcan's Glory is a great book. She manages to draw a fully-realized Enterprise crew out of folks we only saw in one episode. Her style is a joy to read, too; clean and lovely, with a bit of poetry in it where it's needed.
A guilty pleasure is the team of L.A. Graf. There are some issues (the occasional Mary Sueing), but they do a brilliant job of tying today to the future of Trek. And they write action/adventure really well, too. Plus, it's nice to see a team that focuses on the bottom three of the main bridge crew, instead of Kirk, Spock and McCoy. I love how they write atmosphere -- they make you feel the rain, the cold, the actual places you walk with them.
There are also a whole slew of fanficcers out there that I think really nail it, and well. Who spin brilliant yarns.