It is famous for the Sultan Ahmet Mosque, also known as the Blue Mosque because of its interior blue Ottoman tile works. This imperial mosque built in 1609-1616 is an example of classical Turkish architecture and it is the only mosque that was originally built with six minarets. I do not particularly like going there in the day as it is always crowded, but its the best time to have its photos taken. But I do enjoy the countless quiet moments that I have spent sitting on one of the many old wooden benches just in front of this majestic mosque.
There are, of course, other places to visit like the Basilica Cistern, Hippodrome, Grand Bazaar, Egyptian Bazaar, Suleymaniye Mosque, Ortakoy, Pierre Loti, the Bosphorus, Dolmabahce Palace and the list goes on. All these are a must see. But the most important thing for me when I am in Istanbul is to just sit among locals where I can enjoy a cup of tea while witnessing the sun setting beyond the Blue Mosque and the Topkapi Palace. Away from the hustle and bustle of the other tourists. Drop me an email if you want to know my secret place :) but then again, it might not even be a secret place anymore.