Union Hotel Karakoy is a great hotel in the heart of Karakoy. The building is newly renovated and looks beautiful. The staff are really what set the building apart though - the professionalism and quality of service were excellent. We interacted with Casim on a few occasions and he went above and beyond for us each time. The team is knowledgeable and attentive, providing info about the city and its many tourist attractions worth visiting.
The location is great, being close enough to walk to nearly all major sites you'd want to check out in the city, including it's incredible food scene. That being said, it's on a fairly busy main road, which means the street traffic can be a bit loud overnight even with the windows closed.
Overall, it's a great place to call home for a short stay in Istanbul, with wonderful resources as your disposal while there.