The Turkish Airlines Business Lounge in Istanbul is the best I've experienced. Its sheer size, sleek design, and, most impressively, the variety of Turkish and international cuisine make it a destination in and of itself. The amenities include private suites, showers, and even a golf simulator.
The penthouse-level pool is accented by in-water loungers and a surrounding sun deck, offering stunning, unobstructed views of the city skyline and iconic monuments.