Main floor . That is where you spend most of your time . Running up or down 2 x for every load gets pretty frustrating quickly. BTW a W/D in clothes closet is never ideal Think one big load down and one big load up from the main floor. I have had all 3 and I love my main floor laundry

BUT if they're in the master closet, they should be a stacked pair. The only way I'd be OK with them in the basement is if the house had a laundry chute. I don't mind the machines there, but I wouldn't want to be carrying the laundry there.

I bought a house with a stacked washer/dryer in the master bedroom closet. I love it.

This house was remodeled several years ago with a standard 3-bedroom 1-bathroom wing changed to a master bedroom, master bathroom, and half bath in the hall. The master bath is very large and so is the laundry closet. Perfect for us.

What's on the other side of the wall behind or on the sides of the washer and dryer?

Behind the washer/dryer is another closet. to the left is the hall bath, and to the right is the exterior wall