They're not really on the tourist trail. Many of these countries are either considered too dangerous, too distant or too pricey to even consider for your holiday plans.
However, reputations aren't everything, and some destinations are worth a bit of hassle. For something a little different on your next trip away, this is where you want to be heading.
I had to Google Azerbaijan before I went there to find out where it even was. There's very little known about this surprisingly rich country, "the land of fires", a hugely diverse place perched between Iran and Russia, enjoying Caspian coastline, lush foothills and high mountains, and a booming economy propped up by oil and gas revenue. You can window-shop for Lamborghinis in the capital, Baku, and then be wandering through a tiny country village a few hours later. It's… bizarre.
Papua New Guinea
One of our closest neighbours is also one of our least-visited destinations, a wild place oozing culture that's cut with a dose of danger. There's some of the best scuba diving on the planet up in PNG, as well as events like the Mask Festival in Rabaul, and the Canoe Festival in Milne Bay. There are also the cultures of hundreds of tribal groups who speak more than 800 different languages to explore. That should keep you going for a while.
It's not going to be easy getting to Mongolia. And when you arrive in the capital, Ulan Bator, you'll be pondering the wisdom of making all that effort. It's not the nicest place. Out in the countryside, however, is where Mongolia really shines, a scene of rolling hills dotted with gers, of nomadic tribes still getting around on horseback in traditional clothing. It's one of those places that'll leave you gaping.
You have to really want to go to Portugal. You're not going to call through there on your way to somewhere else. You're not going to be lured by famous monuments or world-renowned cities. If you do visit, however, you'll find an amazing country of beach-resort towns, winery-laden hills, bubbling, lively cities like Lisbon and Porto, and some of the best food in Europe. Two words: Portuguese tarts.
It doesn't have Tanzania's Serengeti, or South Africa's Kruger, but what Namibia does have is a surprisingly rare commodity in southern Africa: peace. In this rugged country of only 2 million people you can drive for days without seeing another soul. Eventually you might bump into some Himba tribespeople, with their mud-caked hair, or a community of Hereros, with their wide, flat headwear. Head to the coast, meanwhile, and you'll find huge desert dunes and the adventure capital of Swakopmund.
Much of Central America remains rarely visited by Australians, given the effort required to get there. But it's worth the trouble. One of the most diverse and interesting countries in the region is Guatemala, where the buses are colourful and often clapped out, the volcanoes are smoking, the rum is strong and the people are friendly. There's also Lake Atitlan, a volcano-ringed body of water that might just be one of the most spectacular places on the planet.
Another country that's often overshadowed by its more famous neighbours – in this case the likes of Peru, Chile, Argentina and Brazil. But there's nothing in those countries that can match the sight of flying into La Paz, a huge city sprawled across the high Andes at nearly 4000 metres above sea level. Outside that great town you can visit the world's largest salt lake, Uyuni, or the world's highest navigable lake, Titicaca. Or you can head to the Amazon basin in Rurrenabaque.
The bigger cities, places like Yangon and Mandalay, are no longer off the beaten track. Take a look around – check out all of the tour groups getting off their cruise ships and filing around the major sights. Myanmar is on the tourist trail. Steer clear of those cities, however, and you'll still discover a culture and a people that have been cut off from the Western world for a long time. It's also one of the most photogenic countries you're likely to find.
I know, I know – I'm writing about Iran again. But have you been there? Maybe not. And should you? Definitely. This is a country that defies expectations, that batters you with an onslaught of friendliness and goodwill. It's safe to travel around, it's beautiful to look at, the food is great, the tea even better, the prices are cheap, and you're absolutely guaranteed to make friends. You can't ask much more than that.
It's not cheap in French Polynesia. It's not cheap to get to (given it's plonked right in the middle of the Pacific Ocean), and even if you choose to do a homestay on an island like Moorea instead of an overwater bungalow stay on an island like Bora Bora, it's not cheap to stay. It is, however, a stunning set of islands, as remote as you could hope to find, with culture, French-influenced food, and all of the palm-tree-and-white-sand clichés that you could poke a coconut at.
Which do you think are the countries most people haven't visited that they should?