(i'm a assuming fan-fic writers count)
1)Andorians and Klingons.
2)I'd love to write a Caitians book and take them beyond being a bunch of cat-joke aliens. (PAD, I'm looking at you...)
Romulans, personally. There's something almost Shakespearean or Roman about them that feels different than writing for other species. But, lately, I've taken a shine to the Bajorans. Again, it's like writing for humans in a different point of history; similar to yet different from the perspective of the reader and the writer.