Which neighborhood to stay in in Istanbul
 

What are you most interested in doing when you are in Istanbul? Do you want to be close to the tourist attractions or not?

Karaköy. Overlooks the water. Easy to cross back and forth to sultanahmet where traditional tourist attractions are. Lots of hip restaurants in walking distance.

As others have mentioned, it really depends on what you're going for. You can also split time, 2-3 days in Sultanahment for the tourist attractions followed by 2-3 nights in another neighborhood like Nisantasi/Sisli or Karakoy or Besiktas. If you want to check out the Asian side, I'd recommend Kadikoy (of course I'm giving the English version keyboard names of the neighborhoods).