For myself, Taking Wing and Orion's Hounds are tied for first, with Red King and Sword of Damocles being tied as well.
Taking Wing of course is the first novel in the series and as such, I had low expectations. I only knew Riker and Troi going into this and I figured the first book would be rather like the first seasons of Trek. Boy was I surprised. I loved the look at Romulus (part of the reason I ended up buying Rihannsu so I could get more Romulan stuff) and all their problems and Riker being a mixture of both Kirk and Picard captain styles.
I had a similar worry going into Orion's Hounds. The third book in the series and its not written by the team that wrote the first two, I was worried that there hadn't been enough books to set the tone for a new author to come in on the scene. And the whole story being about the troubles of an alien not seen since the first episode of TNG, my hopes were real low. And again I was shocked at how great this was. The science and action, and all centered around an eco-friendly message.