At the risk of this coming across as a 'one up the rest' post with something so open-ended, http://www.class-central.com/ http://www.class-central.com/ would be

At the risk of this coming across as a 'one up the rest' post with something so open-ended, http://www.class-central.com/ http://www.class-central.com/ would be my choice. They aggregate MOOCs from the top providers, with a search capability.