If we had to pick one, I go with Adam LaRoche.
End of the day it is a complex issue.
Adam LaRoche carried us early in the season offensively. Had he come out of the gate slow, we don't make the playoffs. Throughout the season he provided timely hitting and was a guy who was a lineup stalwart. He was an experienced voice on a young team. Is he likely to repeat this season? Probably not. Is he capable of having another gold glove year where he plays 150+ games, hits 25+ homers, and is good protection for Zimmerman? Yes. Most certainly yes. The Nats need to offer him a competitive contract. Low balling him after he played such an integral part would start the offseason with a very bad taste. If he is willing to return on a reasonably well paid yet short term contract, you bring him back.
Michael Morse has become extremely under valued on this board. He is cheap. He was a huge piece on the 2011 team that really was a sign of things to come. He is two years off a 31 homer, .300+ year. Those guys aren't easy to find. This past season started off with an injury and he still finished with a respectable .291 average and 18 homers. Is he a good LF? No, but his bat compensates. Morse has tons of power. He is explosive at the plate. He can play 1B if needed. If he comes back next year there is a good chance he could hit 30+ home runs. Some people are treating him like a liability, but he is anything but. He carried the 2011 offense and then this year was the best left fielder on a 98 win team.
If you move Michael Morse, it has to be a solid return. If you let Adam LaRoche walk, you have to be prepared to bring in a quality CF (move Bryce to LF) so that Morse can move to first base.
I like Tyler Moore, but it is crazy to think that he is ready to be an every day player on a team that should be aiming for 100+ wins and a championship. 263/.327/.513 with 10 homers by a 25 year old profiles as a valuable bench bopper, not a guy you want to make a starter.
Letting Morse or LaRoche walk with Tyler Moore being the replacement plan is a step backward. We're trying to win a championship next year. If we let quality walk, we need to bring quality back.