Which alternative energy source is the most cost effective?

Congratulations!  You are being considered for a paid internship at the National Renewable Energy Laboratory.  The hiring committee is asking you to submit a report about what you consider to be the "best" renewable energy source in terms of cost effectiveness.  You'll need to expain what factors you are including in your analysis, both short-term and long-term costs involved, examples of current or future projects, and benefits and problems involved with developing this energy source for residential and / or commercial use.  Feel free to include your thoughts and opinions, but be sure to back them up with research!  All sources must be properly cited, and any assumptions you make should be clearly explained.  Keep in mind throughout this whole project that you need to clearly address the committee's main concern at this point: "What factors should be considered when determining the cost effectiveness of a renewable energy source?"