Which of the main six would be most likely become a parent?

The next generation perhaps? ^^

Pinkie Pie. Become a parent only once or a few times and keep the foals, though, Applejack.

Fluttershy, Applejack and maybe Pinkie.

Applejack strikes me as the one who would most want some sort of offspring to carry on the family traditions and inherit the farm someday.
 
I somehow picture it being Rainbow Dash who’d be the first to have one though, just because she strikes me as the one who’d want one the least, and because life is funny like that at times.

Fluttershy or Applejack

@Flutterio  
for twilight it will be her toughest challenge ever

@Background Pony #A1EC  
I think after meeting Flurry Heart, Twilight’s probably gotten herself spayed.

After seeing ‘Filly Vanilly’ Big Mac?