* The most contentful working time for me was when I had a co-founder. It was good to bounce ideas off someone, and having another person with equal share in the business increased accountability and my motivation to keep pushing.
* The most effective working time for me was as a solo-founder. I felt that I wasn't compromising every decision I made to keep my co-founder happy, and that I could make decisions much more quickly and change direction whenever I wanted.
I think the main problem with the time with a co-founder was that we were too similar. We worked great together and got on well, but there wasn't that spark that pushed the business forward. Since being on my own again and pushing against the limitation of my own personality and abilities, I have managed to grow the business trifold since my co-founder left.