A difficult category here... Very vast.
If pressed for a race I like to use best, I'll go with the skythians from Saggakar. Created and bred only for war, they since evolved and are trying to come to terms with their nature versus what they discover of the world.
Conflict and story potential in one.
This makes the fun to write for as their inner nature is to give in to mayhem and slaughter, but they have more. In some way, I see them as teens trying to find themselves, who they are and what their role is in the world. As a writer, I have a lot of freedom with them. Some may go one way, others another all the while keeping to who they actually are without changing or creating a bunch of oddball character concepts.
Fun to write about, fun to play. I like 'em!