Alone, unless it was necessary to work as a team...by my standards. When I was teaching we would have periodic faculty meetings where we split into groups of 8 or so and we were given stupid tasks to perform as a group. Most thought them to be very tedious and few had any interest. I would usually just sit and draw stuff.
I work fine by myself and usually do OK in a team. However I work best and enjoy it most in a two-person team as long as the both of us have similar goals and work styles. It doesn't matter whether I am the lead or the follower or whether we are equals. Currently, I do volunteer work where there is one paid staff member and myself handling the job. I love how we keep each other informed and can seamlessly pick up where the other left off.
I prefer to work alone. When you work in a team, everyone has their own idea of how to accomplish the "mission." You spend more time deciding on "how to do" than "doing."