I lost 2.5 stone over about four months when I was in my early 20s. My starting weight was 11 stone 5 and my lowest weight was 8 stone 10. My BMI went from 25 to just under 20. I did it by making a conscious effort to eat less, and swimming most days.
I think the very lowest weight wasn't really sustainable. I wasn't underweight but I couldn't really relax and enjoy myself. My weight has crept up again. A few years ago my weight was consistently around 10 stone 3 and I found I could stay there without too much maintenance. For the last couple of years I've had a healthier diet, less alcohol and more exercise and my weight has pretty much stabilised at around 9 stone 10, except when I lost about half a stone just before my wedding and then put it back on again.
I think it's quite helpful to lose a lot of weight and doesn't necessarily matter if you put some of it back on again, if it enables you to find what is a comfortable weight for you, taking into account your lifestyle, how much you want to watch what you eat and drink and how much exercise you're prepared to do, versus how you want to look. That won't be the same for everyone (someone who really loves their food might be entirely comfortable with weighing a bit more than someone else of the same height and build who really loves clubbing in skimpy dresses, for example), but as long as you're in the healthy weight range and feel good about yourself, it's all fine.
But if you do an unsustainable crash diet and lose an enormous amount of weight by severely restricting what you eat, you'll only put it all back on again and fuck your metabolism up in the process. So, don't do that.