.edu and .gov tend to be the most reliable domains. A .edu domain means that an educational institution and .gov is used for governmental agencies.
The .org sites are the second most reliable, since an organization has put forth the time, effort, and money to not only start the website, but to maintain it as well, it seems reasonable that they would use factual information. However, since some organizations have been known to use false propaganda, it is usually best see if the same information is on one or two websites.
The .net and .com. These sites have been bought, and the buyers can put just about anything on them. So if information is used from these sites, it is best to check with three other sources of information.