I have worked on Houston, Flounder, Stargazer, Expound and Twenty Fourteen. I am going to try some other themes from this list as well. These are all great themes. I particularly loved Stargazer because I am a huge fan of Justin Tadlock's works. I also loved Expound for the way it handles featured posts, it was perfect for what I was working on. I used Expound with the Video Thumbnails plugin to create a video site. It was so much fun.

I love free themes from WP.org, they are well tested and usually very easy to modify and build upon.