Although Istanbul has no 'city centre' as such due to its spread over two continents and various hubs on the European Side and Asian Side, which act as centres in themselves, the generally accepted central districts of Istanbul tend to be on the European Side of the city, the zones stretching from touristic Sultanahmet to European centre Taksim and along the Bosphorus toward the city's business & financial districts such as Mecidiyekoy, Maslak, Sisli, Nisantasi, Levent, Besiktas etc.
If you are looking for a holiday home in Istanbul, then the city centre is certainly ideal. It will provide the buzz, entertainment, cultural spectrum, 24/7 activity and lots more. If your budget allows, a Bosphorus view apartment would be as prestigious as New York Manhattan real estate and certainly as prime an investment property as London prime real estate and Manhattan prime real estate.
If you are moving to Istanbul with work or in order to set up a business in this vibrant city, then our advice would be to stick to Istanbul City Centre for your permanent home in Istanbul. If you are going to live in Istanbul, then live in 'real Istanbul' and keep away from many new developments on the outskirts of the city presented as 'Istanbul Property Investments'....you might as well be in a totally different city!