Interestingly, for the past 3 or 4 days there's been a mystery Go Bot playing on 2 servers calling itself "Master" which is currently about 50-0 (!) against professional opponents, many at the very highest level. Nobody knows its identity yet. It's stronger than the commercial engines, and its presence must be some sort of stunt or test by someone.
Diagrams with moves at https://tieba.baidu.com/p/4922688212?pn=150&
Google translate helps with the comments but they don't add much to the diagrams.
They are playing fast games with 3 time periods of 30 seconds per move. It means that one can exceed the 30 seconds limit and move on to the next period. When the limit is exceeded in the last period the game is over.
Fast games usually increase the chances of the stronger player if the difference is large.
Edit: 59-0 just after I posted this.
Strongest open source engine is Pachi, which has achieved an amateur 4 dan ranking on the KGS go server. Fuego is almost as good.