Sociology.
I consider the most useful subjects to be mathematics, and history. Depending on what you do for a living, English and science are also very useful (but not absolutely necessary.) The reason mathematics is important is that it helps develop rational thinking and reasoning (something that most people are in desperate need of). Whether you realize it or not, you use everything you learn in algebra every day. I actually think we don't go far enough, and should include calculus (a subject that use to be mostly college level, but a lot of high schools have it available) as a required class as well. A logic course would also be really cool to have, but it's still considered a college level only course. History is important for obvious reasons.
English is important if you plan on doing a lot of writing (being that basic English from pre-school to middle school is enough to effectively communicate with others). Science (such as biology and chemistry) are important if you plan on doing anything from psychology to engineering.
The most useless courses I would consider would be sociology, foreign language, and skill-based classes (like shop). The reason I consider foreign language to be useless for most people is that they will probably not use them. In my high school, we had 4 levels of foreign language (the first two where required, the next two where not and where college-level). If you didn't plan on taking all 4, I don't see the point in taking the first two. Anything you learn from learning a foreign language indirectly can be learned from other courses much more effectively (like math). I would learn foreign language only to learn the language itself (either because you want too, or because you think it will be useful). I figure anyone can learn what they need as far as a different language if they absolutely had too without having to take a class. Skill-based classes are only useful if that's what you plan on doing for a living (or even just as a hobby). Sociology was probably the most useless course I took, only because I found it to be based little off science and was full of assumptions not based on reality. I disagreed with many of the conclusion made.