The safest and most successful is weight watchers. The food choices are good. Incorporate exercising, consider it a way of life and you will never worry about weight again. Most important have a realistic goal. Try not to say you are on a diet, for some reason over weight people become jealous and try to sabotage your goals. Don't let them

If you truly do need to loose 100 pounds, emphasize the 'effective' part. You are already terribly unhealthy. When I needed to lose half that, my doctor said to use whatever diet worked for me, even if it wasn't a 'healthy' diet.