I have travelled to about 50 countries so more than many people but nowhere near all countries. I am unlikely to realize it but it is nice to daydream, at least, of visiting all of them. A recent question on visas for Saudi Arabia surprised me, I had not known and would not have guessed that they are hard to get. So, I wondered which others are hard to get.
I am a dual UK / Irish citizen.
To count as a visit, I should pass through immigration and stay at least 48 hours. So, merely changing planes won't count and very short transit visits won't either but a more generous transit visit would. So, China is in this category.
I will consider regions with distinct immigration rules as separate countries so mainland China and Hong Kong are separate countries for this purpose. Also, Taiwan is a country for the purposes of this question.
I can attempt to divide the countries in categories.
I can just go there. Most (all?) of Europe for me and quite a few other places.
I need a visa (or visa like permission such as an ESTA) but it is easy to get. So, the USA and quite a few more countries are in this group.
I need a visa and it is not as simple or as immediate as an ESTA for the USA but there is little chance that I would fail to get it.
I need a visa and it could be difficult and there is a significant chance of failing to get it.
Very difficult without some special circumstances.
I am most interested in countries that fall into groups 4 and 5.
My travel history is good. I have never been refused a visa or admittance. I have never overstayed. I have a good job and own a house in the UK.