The only way to really know for sure in advance what your ideal bodyweight would be, is to get a bodyfat test done and see how much fat you have now and how much you need to lose. you could be 5'9" and 220 pounds with 5% bodyfat and look absolutely ripped, and you could be 5'9" and 220 pounds and just look fat. the variable is what percentage of your weight comes from what kind of tissue. and unless you know what percentages you are starting at, and what percentages you want to shoot for, it is hard to give a very accurate answer to what your ideal weight is. Ideally, you should take it slow and steady, 1-1.5 pounds a week of weight loss, and just monitor yourself as you go. when you feel good, and feel that you look good, then you are at your ideal weight.
Based on my experiences, i can give you an estimation for now though. If you are of average build and fitness, i would estimate your ideal weight is around 155, give or take 5 pounds, at 5'9" tall.