A mid size sofa with two chairs and maybe a bence will give you the most flexibility. Its fun to throw a couple ottomans under a console table for seats you can pull up when needed.
Layer in smaller chairs in the perimeter to fill out the space. Remember to maintain your focal points by pulling furniture in and around the fireplace and windows.

Agreed. Whatever you do- don't put a sectional in front of those windows! A couch with 2 chairs facing it with a coffee table in the middle will be a nice sitting area. You could even do two couches facing each other.

here is a better picture showing the windows/fireplace/tv, etc.

You'll have more flexibility with separate pieces instead of a sectional.