It is close though they are all going to be great. Larsson may not be playing on the PP right now, but he is developing real nicely. His 5 on 5 game is improving significantly. I can see him taking major steps forward offensively within the next 2 seasons. Schulz is the best right now, but he is also 3 years older and is on a much worse team.
McDonagh and Schultz played together at Wisconsin (along with Gardiner and Hobey finalist Brendan Smith) and Schultz is only a year younger.
Both players played three seasons for Wisconsin, Schultz leaving two years after McDonagh. I think McDonagh is certainly a valid inclusion given that information.
I don't think OP was ever claiming that this thread was about every young defensemen you could possibly build around. He's asking which out of these 3 we'd pick. It's baffling that this requires explanation.