One monster that I've used a lot in a number of my settings is the hobgoblin (although curiously, not in DARK•HERITAGE. Then again, I don't consider that a D&D setting.)
One of the things I like about them is that they should, by virtue of their description, not really be savage humanoids rumaging around on the fringes of society like so many bandits or tribes of barbarians. If they're so militaristic, well-organized and lawful (ugh. Alignment.) then why don't they have fully civilized, militaristic empires?
There was a picture floating around years ago on ENWorld (it's actually still here; I'll hotlink it here in a moment) called "Pax Hobgoblinica) which perfectly illustrated this exact concept, and I've loved it so much that I've incorporated some aspect of it into tons of my settings, with the exception like I said earlier, of DARK•HERITAGE. But wait! Even that's not so simple. Kurushat, an important component of DARK•HERITAGE actually started out as a hobgoblin empire. I later decided that hobgoblins were too traditional fantasy to belong in that setting, so I made them a human ethnicity instead. But I didn't change anything else about them.
So, hobgoblins it is. Not necessarily because they're really my favorite favorite, but because lacking any other criteria to pick my favorite, I'm going with what I've probably used the most.