Sorry but there's no easy answer to that one. There's no best diet, you have to find the one that's right for you, works with your lifestyle and that you can follow for the rest of your life. So all I can suggest is that you do some research and browse the various forums here and ask lots of questions before you decide what you're going to do.
I wouldn't bother with speed as a priority if I were you. The faster you lose it, the more likely it is that it will all go back on and a bit more too!
If you're a first time dieter, then I would highly recommend Slimming World's Extra Easy - it's easy to follow, and has the benefit of not leaving you hungry all the time. Pick up the latest mag - there's usually a sample menu in there to follow.