The most powerful passports are those that let passport holders enter other countries visa-free. The more visa-free the country, the more powerful the passport. Nowadays we can consider powerful such passports as Japanese, Canadian, American, Australian, German and a few other European countries. However, there are some countries that make it almost impossible to get in. Here is the list of countries, where even the most powerful passport holders might have challenges to enter.
Nauru. In February 2018 Nauru canceled all visitor visas after it alleged an Australian media representative entered the country on a visitor's visa with nefarious intent. The only possible reason for a non-Nauruan to visit Nauru is to peer into the island's people-minding industry.
Saudi Arabia makes it clear that it does not need leisure tourists. There is no such thing as a tourist visa for Saudi Arabia. The only way in for a non-Muslim to enter Saudi Arabia is on a business visa.
Bhutan. Apart from Indian, Bangladeshi and Maldivian nationals, all visitors need a visa, which requires booking your travels through a Bhutanese tour operator or one of their accredited international partners.
Turkmenistan. Turkmenistan does not exactly welcome foreigners. You need a letter of invitation, facilitated by a local tour operator, which requires booking a tour through an accredited travel agent. You'll be met at the airport by your tour operator and shepherded around the country by them.
Gaza Strip. A tourist visa does not exist here either. Visitors need a good reason to enter, such as working under the auspices of a human-rights organization. Since there are no functioning airport visitors must enter the Gaza Strip by land at either Erez, which is in Israel and under strict control or Rafah in Egypt, also under close supervision.
North Korea. Another country where you need a full package of accommodation, meals and tours to obtain a visa, arranged through an accredited travel agent.