I’m no expert… but it seems to me that if you can do butterfly for an extended amount of time, I’d use that.
Most likely freestyle would be your best bet, though. Once you get to the point where you can swim over a mile you can build up hundreds of meters pretty quickly if you keep at it. (or at least I was able to).
@itsnotmyfault1: I think breaststroke is actually easier to swim distance than freestyle is, but I’m not sure because I can’t really do breaststroke right. Backstroke is also easier if you have trouble with the breathing style of freestyle but don’t have trouble with the weirdness of backstroke.
However, I should tell you that I weighed myself just before and just after my school’s swim season (which I was very much a part of) and found that my weight hadn’t changed by a single pound. (to be fair, I did try to put myself on a higher-calorie diet because I’m already skinny as hell, but this consisted of little more than switching to drinking whole milk rather than 1%)
So, in my experience, while it is the best sport in the world, swimming isn’t exactly a weight-loss program.