I'll be honest, I'm not super savvy when it comes to the legions and divisions and corps and all of that other military jargan-esque related stuff.
However, I always liked the 327th Star Corps. Yes, I didn't know their name until I went and Wookie'd it for this thread. I knew Bly and I knew their awesome yellow markings. It was only a matter of a few minutes of searching.
And I bet everyone says the 501st 'cause that's the only one I know by number (or rather, I only one I did know!). Besides, I've actually met the 501st, which makes them awesome in my book no matter what. I even got a picture with them.