I second Swanhunter's recommendation of the Empress Zoe; though I'm not sure whether the lack of aircon will make it a little sticky at this time of year (nice place though and always worth keeping in mind for another visit).

I suspect that the Four Seasons may end up being the one to go for - even though each trip to IST I try to avoid these homogenised places it's guaranteed to be comfortable in the heat (albeit at a premium) and the location is hard to beat.