I like big and black... my go to fly in the winter is a 3/0 black general practitioner tied on as sharp a hook as I can find. If you prefer a simpler tie a 1/0 bunny leech is a good choice. By far the most important thing about fly choice is your confidence in that fly. That goes for the water you fish also actually that goes for all parts of your setup..

take a look at the stuff bob has been tying and posting in the daily reports that will give you the idea...

I like big flies , some guys like small flies it really doesn't matter nor does color pick one you think the fish will like and fish it with confidence. keep your hooks sharp.