I think the strongest commercial engines are Zen and CrazyStone. Zen has certainly become pro-strength on fairly modest hardware (it had an exciting BO3 against a Japanese 9-dan, which it lost 1-2), but the strong version isn't available to the public yet, I think.
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.