Simply, Milford Sound is one of the most beautiful and rugged places on earth, and I still am thinking about it often, months now after our trip there. The mountains are rough and they ascend staggeringly high straight from the ocean, carved by unbelievably massive glaciers churning their way down to the South Pacific. Centuries later, the area is wet and lush, its cold, ferny rainforest more similar to what you would find in the Pacific Northwest than the tropics. Bossy, large parrots roam the streets. There is only one hotel/hostel/rv park (and it's nice).
The drive down to the Sound has to be one of the most spectacular drives on earth. Around each corner on the winding road, craggy and impossibly tall mountains appear, one after another. Water cascades down the sharp faces when it rains, creating innumerable waterfalls and streams. Put it on the list!