It's free, helps you monitor food and exercise, there's a diverse community and you're free to follow whatever food plan you want to.
It's helped me get down to a BMI of between 20 and 21 and more importantly, I've maintained around that weight for over 18 months now.
I'm currently planning to lose around around 4 lbs to get to 9 stone exactly - I've decided to follow BIWI's low carbing regime for a few weeks as an experiment and if that doesn't work for me I'll probably revert to calorie counting and my usual weight loss 'rules'.
I've done all 3 and only WW worked for me, though I've not done any long term and reached a goal. Having said that, I know people who've been successful on all 3 so I don't think one is better than the others, it's what works for you. I like WW because I can have anything I want, just in moderation so it fits into my lifestyle. I don't get on with SW because they allow unlimited quantities of some foods and I don't have the restraint to keep that sensible. Never lost on it at all! Liked the exercise class with RC (but assume you won't get that on line?!) but just couldn't quite stick to it 100%, WW allows more slippage, so long as you point it and make it up and don't do it again in the week! But as I say, any of them might work better for you! I understand Tesco diets are worth looking at for online only too. Good luck!