The best view in Istanbul costs the price of a fare. The public ferries pass everything the tourist boats pass, they are paid for with the İstanbulkart, and they are full of people on their way to work.
Karaköy → Kadıköy. The nearest to us: seven minutes downhill and you are aboard. Twenty minutes across, with the Topkapı and Hagia Sophia skyline to starboard and the Maiden's Tower to port. On the far side, walking to Moda — or timing it for the Tuesday market — makes a good afternoon.
Eminönü → Üsküdar. The shortest and busiest line. Ideal for ending a day on the Historic Peninsula by crossing over; from Üsküdar you can go up to Çamlıca or walk the shore.
Beşiktaş → Üsküdar. Short, but it crosses where the Bosphorus opens widest.
The long Bosphorus line. Eminönü up to Anadolu Kavağı, about six hours there and back. Touristic, but close to public-ferry prices. Worth it if you can give it a whole day; if you cannot, the Kadıköy crossing already gives you the best part of the same view.
The hour. Board half an hour before sunset — about half past four in winter, around eight in summer. The upper deck is windy, the lower one glazed and warm; look at the weather and choose.
Practicalities. Tea and a simit are sold on board, and buying them is one of the things to do in Istanbul. It is true that the gulls expect the simit.