Ok - it's actually Mount Ngauruhoe in real life. By either name, it's amazing!!! The South Island gets all the attention, but this North Island attraction ended up being our favorite of the trip.
We hiked (scrambled?) to the summit of Mt. Ngauruhoe. It's an amazing hike. The last section, from the saddle between the mountains to the summit, is unmarked and is simply scrambling up the side of the volcanic cone, with loose volcanic rocks underfoot. The volcano last erupted in 1975, and the area is still quite obviously volcanically active as there is hot steam escaping from the earth near the top. Once you get up the steep scree slope, you'll find an outer crater rim around a crescent-shaped crater. The views from the rim are amazing as are the active steam vents. Then scramble up to the inner crater rim on the right. From there you can see deep circular crater. Note - do not try to enter the inner circular crater area as there are dangerous volcanic gases.
Mt. Doom made it to the top-6 list because it was such a unique landscape and such a unique way of experiencing it. The area is quite geologically active, and the earth newly formed and raw. The rocks were amazingly colorful: deep blacks, whites, oranges, and reds. The climb up the steep sides was a real challenge, and the reward at the top was unbelievable.
Mt. Ngauruhoe is on the North Island in the Tongariro National Park. You can access the mountain as a side trip from the well-known Tongariro Alpine Crossing walk or simply as an out-and-back from the Mangatepopo parking area. We were short on time and hadn't pre-arranged a shuttle, so did as an out-and-back hike. If you do it as part of the larger crossing, don't be tempted to skip the scramble to the summit as it was definitely the best part!
Detailed information from the NZ Department of Conversation is here.