Istanbul is a fairly large metropolis. It is hard to capture the full orientation of the city from one point but (Buyuk) Camlica Tepesi is the highest hill in Istanbul and it would probably be the best option. It is accessible from Uskudar via public transportation or a short cab ride. Other than that the roof restaurants/bars of high rise hotels in Taksim square (such as the Marmara) usually have nice and wide views of the city.
