All of them have valid reasons to get captured based on they're backgrounds and genetic make up but who do you think an enemy will try to capture first Mitsuki, Boruto or Sadara?
The enemy would try catch Sarada first because kekkai genkai are gold in this world but they wouldn't get far. Boruto as a ransom for the Hokage is also interesting and possible, but then who would try threaten the 6th Hokage? A main villain.
The most likely to happen is Boruto before the timeskip happens, plot-wise. Not sure what anybody would want with Mitsuki.
Mitsuki being able to go into Sage mode is worth snatching up especially since he's a lone whos to stop anyone from repeating the process or just use him for personal gain