As is the case with most NFL teams, there are plenty of solid choices here, but the Vikings have more on defense than your average squad. Speedy receiver Stefon Diggs has been excellent for the Vikings in 2015, linebacker Eric Kendricks would give the Falcons a pass rusher and plus athlete, Everson Griffen has 6.5 sacks, and defensive tackle Linval Joseph is an aggressive presence in the middle. if age and off-the-field concerns don't matter to you, Adrian Peterson is still one of the finest running backs in the NFL, as well.
I'd go with safety Harrison Smith, myself. He's a big hitter, a truly well-rounded safety, and a productive player who would be an upgrade over anyone the Falcons currently have. It goes without saying that Atlanta could stand to add more talent on defense, even if the defense has improved.
Who is your choice?