Of course, you are only eligible for a single passport, so the question may be paraphrased. Which citizenship is the most practical if you want to avoid border hassles?
My precious red passport
Some people have dual citizenships and some may have no valid travel document at all. Nonetheless, there is a difference between holders of passports when it comes to visa regulations. I once wrote an article about my passport stamps, stating that my Norwegian passport has always been one of the best passports to hold. Flashing the red-covered booklet has been a door-opener virtually everywhere I have been.
Now, a mapped infographic produced by the magazine Good.is reveals that Norway is not on top, but very near. (Link is now dead.)
The most lucky, and the rest
People from Finland, Sweden and the United Kingdom are granted visa-free access (or receive a visa upon arrival) to 173 countries. I will easily get into 170 countries. If you hold an American (US) passport, 172 countries will open their fences for you without much bureaucracy.
The least welcome citizens are from Eritrea, Pakistan, Somalia, Iraq and, at the bottom, Afghanistan. If you are an Afghan, only 28 countries will let you in without a visa.
The coloured map shows that we live in a biased world. The darkest colours are the “best”, and they correspond to the rich Western democracies and a few others.
Update in 2020
The above was written and published back in 2014. What’s it like now?
There is a website taking on an official look, or at least with an aura of authenticity when it comes to the comparison of passport strengths. Check out the Passport Index.