Sky Pillar in RS was perfect. Fairly out of the way, so you needed to explore to find it, had a challenging but doable puzzle to reach the end, and with a cool prize at the end to battle and capture. The feeling of accidentally finding this huge tower and discovering a super powerful dragon at the end is, along with DP Giratina and BW Kyurem, something that I couldn't find in XY, ORAS, and SM.