In general, it depends on the kind of website you want to build and your target audience. In most basic scenarios .org is not a bad idea, its an established gTLD, but if you are building an online shop lets say, it simply won't cut it for that purpose.
Then comes the audience. Buying a country specific TLD like .us (altho it doesn't have the restrictions of many ccTLDs) can be justified if your business operates in the US and that is where your visitors are coming from too.
In any ways, if the .com of your domain name is taken, choosing a different extension is not such a great idea as you are facing much more challenges like researching any possible copyright issues with them, losing traffic to the much more popular extension etc. So you might still be best looking around or a good .com to begin with.