I don't know which would be most underated but Swiss, Belgium, Serbia, Chile, USA or Croatia are good candidates. Some of them get no recognition besides good results(Chile, USA) and others got worse results than their squad is capable off(Belgium, Serbia)
Among those choices I said Sweden. A 3rd place finish and a Runner up at a World Cup is nothing to sneeze at. Not to mention they just beat Holland. On their day, they can beat anyone and be a big dark horse. On the other hand, when they're in poor form, they can be miserable (see: 2010 WC Qualification). Denmark is also very underrated as well (They do have a European Championship, after all)
If we are talking about only the past decade, I would say Greece. They are the only nation in FIFA's top 10 ranking that isn't talked about endlessly. They have qualified for most major tournaments starting after WC 2002, sometimes convincingly, despite not having the advantage of being a top seed in qualifying tournaments.
Uruguay, at the same time overrated for the obnoxious fans who keep on stressing that winning a world cup in the last ice age and stone age is important. Now that I get that out of the way. They really are overachievers as a country with just 3,5 million people. They have that Southern American passion but also a really cool calm team mentality. I really don't see any European country with the same amount of inhabitants copying that, hell countries with several times the inhabitants can't even do it. I don't see Uruguay as a contender for the next world cup but at least they will give their opponents a run for it's money and don't just roll over.
Uruguay Campeón Olímpico 1924 y 1928 Uruguay Campeón del Mundo 1930 y 1950 Uruguay 4to. puesto en los Mundiales 1954, 1970 y 2010 Uruguay Campeón de Campeones del Mundo 1981 Uruguay Campeón de Amèrica 1916, 1917, 1920, 1923, 1924, 1926, 1935, 1942, 1956, 1959, 1967, 1983, 1987, 1995 y 2011 Uruguay Campeón Sudamericano Sub-20 1954, 1958, 1964, 1975, 1977, 1979 y 1981 Uruguay semifinalista en los Campeonatos Mundiales Sub-20 de 1977, 1979, 1997 y 1999 Uruguay Vicecampeón Mundial Sub-17 en 2011 Peñarol Campeón del Mundo 1961, 1966 y 1982 Peñarol Campeón de América 1960, 1961, 1966, 1982 y 1987 Nacional Campeón del Mundo 1971, 1980 y 1988 Nacional Campeón de América 1971, 1980 y 1988 Población de Uruguay: 3.415.920 hab.
By the way, being underrated and being overachievers are not necessarily the same thing. You can be an overachiever because you overcome bad odds (small population, bad infrastructure, bad management) but at the same time be overrated. A team's rating should come from performance where every country puts 11 players (plus some bench players) on the field. Period. No other circumstances such as the bad odds that I previously mentioned should be included.
How is Ghana not on the list? The second best Nation is African Cup of Nations history, wins and medals in every youth competition and the 6th best nation in the last 2 World Cups. Meanwhile they are lingering somewhere in the mid-30s of the FIFA ranking system, while Uruguay, the team they outplayed in the quarters of the World Cup, is a top 5 country. And despite being consequently one of the best teams in the last 5 years of World football they aren't treated seriously. So my vote goes to Ghana. And then to Serbia, albeit their performances in the last 5 years really have hit their position in World football. --- And why the hell is Togo and the Baltic states on this list?
What's that "fact" based on? They haven't beaten a good team since Serbia some 18 months ago. And even Serbia aren't that great.
What about the USA? They also beat Uruguay after dominating the game (without their two best players I must add). But well... Uruguay cheated. Based purely on the Ghana vs Uruguay game the Africans have the better team, even when they aren't playing full strength.
This Greece has been fairly consistent the past 10 years. And yet so many people outright dismiss their 2004 victory as a fluke or boring football. Based on the results they've produced vs the lack of hype or respect, I'd say Greece easily is the most underrated. Heck, Greece should be ranked higher than England.
The Greeks were thoroughly outplayed by the South Koreans in the 2010 World Cup, and I don't think people would say that the Korea is an underrated country. So I don't see how you can say that Greece is underrated. If anything, Greece is one of the most overrated countries in the world.