The hardest part is keeping it off long term, losing weight is pretty straightforward - any diet will work in the short term, I'd say diet is more effective than exercise personally.. (and based on a number of studies)
The relapse rate of people who've lost weight, who regain it is somewhere between 70 and 80% (usually within a year) and my opinion on this is mostly down to how people become fixated on counting calories and nothing else. Essentially - they don't fix the root cause (high calorie bad diet) and try to eat less calories of the same bad diet they had before, this works temporarily - but eventually for a lot of people, willpower is easily defeated and they go back to the same bad habits that caused the problem in the first place. This doesn't apply to everybody - some people can manage it, but the majority cannot, most of them relapse and go back to being overweight / obese, because they simply cannot resist the temptations, (which are frankly ridiculous in today's environment)
The problem with counting calories and nothing else, is it doesn't take into account the root cause of why someone is overweight. For most people - it's eating too much junk and processed food, which is extremely calorie dense. But the fact it's calorie dense isn't the main problem, it's how the junk food has been designed (Eg; Pringles) which is where the problem lies. A steak is calorie dense but it's very filling and satisfying to eat, (mostly because of how protein affects the body's hormonal satiety response) however a tub of Pringles (1000cal) feels like you've just eaten a tub of air.
For me personally, based on everything I've read - for long term success, it starts and ends with diet, you cannot outrun or under-eat a bad diet and expect good long term results and a healthy strong body.