We stayed at the Hilton Istanbul many times, most recently in April and it was wonderful. The rooms all have balconies with great views of the City. This may be the original overseas Hilton but it shows it's age well and the service and the lounge was great. The location near Taskim made getting around the city easy.
Be aware that the relatively isolated location and size makes it a favorite of diplomatic meetings. Every time I was there there has been some big international conference with lots of security.