Whatever you think will be the most motivational for you. Clear as mud, right?
I've seen 3FCers set high goal weights (something like 10 lbs down, 10% of their body weights etc) and I've seen others who set their goal weights at the low end of a healthy BMI even when they have 100+lbs to lose. You do what works for you.
Personally, I've always set a range for myself. I definitely want to be at a healthy BMI so doctors have to leave me along about my weight! Beyond that I'm not sure. I'll be at the very top end of a healthy BMI if I lose 9 more lbs and after that I'll just keep reevaluating to see where I want to stop. I have some non-scale goals too. Ultimately I'd like to have a 28" waist or under (not sure if that's going to happen, though...) and be more or less happy with my appearance. For the most part I'm there already except my midsection so I figure I'll continue to lose until at least the high end of my BMI and then decide if it's worth it to go further or not.
The nice thing about a goal weight is you can always change it. If you don't feel happy there go lower. I also highly recommend mini-goals because it helps so much to have a number that you've been looking forward to hitting along the way!!