There is nothing wrong with either brown rice, baked potatoes or couscous for the purposes of losing weight, all you have to think about is being under your maintenance for the day...you see:
Your body requires a certain amount of calories to maintain it's current
weight (body weight in lbs x 15)...so if you were 150 pounds, that would be
150x15 = 2,250 calories/day.
To lose 1 pound you need to be in a deficit of 3,500 calories. So if
you ate 500 calories less than this 2,250 each day (1,750 cals/day) you would
lose 1 pound each week.
I would guess though that if you were to compare 1/2 cup of brown rice, couscous and potato, that the least calorie dense item would be the brown rice.
So in terms of satiety, you might want to go with the brown rice out of the rest of the options..but really you could have any one of them as long as you counted your calories and ensured at the end of the day you hit your calorie goal. If things are still unclear &you need more info,
there's more detailed info on all of this here..