The problem here is that those numbers are meaningless when trying to determine "best" - because we're all individual and we all have different fitness levels.
What's BEST is the amount of exercise that gets your heart rate up. As far as losing weight, many studies have shown that interval training is the most effective. Interval training is where you get your heart rate up as high as you can go for a short period of time (most people can do 20-30 seconds at that intensity) and then lower it back down to a mid-level intensity for about 2 mins. Then repeat the cycle again for about 20-30 mins.
If you can't/won't do that kind of interval training, then making sure you get your heart rate up to a medium intensity level and keeping it there for at least 30 mins is important for both building cardiovascular endurance and for losing weight.
What numbers on the tread mill get you there - that's totally up to you!