Depends on the country, obviously. I do not know of any country where you don't have to go through a complex administrative procedure that takes months or years. In places where social services are very good (say Switzerland) it is much harder to get citizenship than where they aren't. If you're planning to receive benefits for the rest of your life, researching the prerequisites for various countries should be well worth your time.

The easiest way is always the same, however: Convince a citizen of that country to marry you. Many countries have discrimination policies where it is harder to acquire citizenship by marriage if you're from, say, Nigeria, but coming from the US it'll be easy in most places.