A crash diet is just that. It makes your metabolism crash and screws you over in the long run. Losing weight fast causes your body to work harder to retain stored fat, meaning the second you do eat a little bit more and aren't starving yourself, you're likely to put weight on again easily and will end up heavier than before because your metabolism is so messed up. Weight cycling is also terrible for your heart if you keep doing it.
Personally, I don't diet and I've never really intentionally tried to lose weight even though I am a bit overweight, but the only time I did lose weight easily was eating a paleo style high protein, high fat diet with limited carbs. I didn't eat that way to lose weight, more just for health reasons as I think it's probably healthier and I was starting to feel fairly unwell if I ate sugar or a lot of carbs. So I decided to see if it helped to cut down on carbs and processed foods and replace them with protein and fat with lots of fresh fruit and veg. I felt a lot better right away so kept it up, though not strictly. I ate pretty clean during the week, less so on the weekends (but not ridiculously so) and I didn't stress if I went to a friend's for a meal and they gave me some potatoes. I was also trying to work out about 3 times a week, but I didn't always manage it. I lost about a stone without trying over several months. It is more of a lifestyle change though (what I would class as 'healthy eating') rather than a diet. You won't just magically maintain that weight loss if you decide you're doing to eat cake every weekend. I think there are no magic quick fixes unless you want to eat healthier and stay active.