Choosing a college is difficult. While there are plenty of resources to help students estimate hard costs, it can be hard to understand which features make one school a better fit than another.

The U.S. News & World Report annual ranking helps make this process a little easier. The publication has long tracked and analyzed data from hundreds of schools, studying metrics such as graduation rate, retention rate and class size. For the first time, the rankings will also consider a school's ability to promote the social mobility of low-income students. The list also features a four-way tie for third place.

Once again, Princeton University topped U.S. News' rankings of the best universities thanks to a perfect 100 out of 100 overall score and an impressive five-to-one student-to-faculty ratio.

Common among these top-ranked institutions are sky-high first-year retention and graduation rates. These figures are often overlooked by prospective students but are incredibly important when considering colleges.

The National Center for Education Statistics reports that just 40 percent of first-time full-time students earn a bachelor's degree in four years, and only 59 percent earn their bachelor's in six years.

Since students often struggle to graduate on-time, they are forced to pay for extra years of college which can quickly increase the cost of their degree.

While not all students will attend one of this year's top-ranked 6 schools, understanding the traits these institutions share can go a long way toward selecting a university that serves them best in the long term.

With these factors in mind, here are the top 6 schools in the country, according to U.S. News & World Report:



