Bambook is unlike any guesthouse you'll ever stay in. The owners, Wes and Lynn, are great people who will go out of their way to make you feel comfortable and at home. The property itself is nice and has a laid-back ambiance, as well as a great view of the Black Sea from the balcony. You can get coffee, tea, breakfast, and alcoholic drinks at the house, and you can bring your own stuff to cook in the kitchen.
This is one of my all-time favorite hostels. The owner, Ahmed, is a really great guy who is always willing to help out in any way he can. The staff is usually friendly, the building is old but has a really comfortable, homey feel, and is always clean. I've stayed here several times and there have always been interesting and cool people there. It's located right on Istiklal, a couple blocks down from Taksim, so the location is 100% ideal.
This is a very nice hostel with a very friendly and good-looking staff. It's a short downhill walk from the Spaccanapoli, the town center, the Via dei Tribunali, etc., and a straight shot to the seafront. The bar and outdoor areas are great, the wifi connection is strong, and the showers are clean and nice. And did I mention how good-looking the staff is?