We had 43 species at our Great Falls CBC (my first CBC!), and as Marshall pointed out, the Barred Owl was definitely the highlight. Though at one point a pesky Downy Woodpecker tempted the owl to have a snack, he mostly just sat still, basking in the sun.
I also had good views of two Common Loons. One was swimming solo in the middle of the Potomac, and the other was swimming with a Common Merganser, the two apparently good enough buddies (mates?) to want to stay near each other for upwards of 5 minutes.
(Oh, and as a bonus, here's a Downy Woodpecker pecking the ice-covered tree in our backyard.)