If you ask me, it's relationships. It doesn't matter how great your job is, how much money you have or how good you look, relationships play the biggest part in our pursuit of happiness.
In 2002 Ed Diener and Martin Seligman conducted a Positive Psychology study on students. They discovered that the happiest individuals where those reporting "strong ties to friends and family and commitment to spending time with them" ("The New Science of Happiness," Claudia Wallis, Time Magazine, Jan. 09, 2005).