If we only count the characters that have already been featured or mentioned, it's really hard to logically point out that who is The Boss, everything so far is just pure speculation. I doubt that Gosho would leave behind some direct hints like the Vermouth arc, Kir arc and Bourbon arc... Of course except for the known clues like the Boss' identity has been mentioned in some form or his/her favorite in Vermouth... but they don't really narrow down much.
So far, the most suspicious known characters in my opinion are:
• Sharon Vineyard's husband (if he really exists and is not another alias of Vermouth): If he exists and is still alive, then he's most likely to be The Boss.
• Kurisuke Agasa (Prof. Agasa's uncle who died 50 years ago): I don't really know why I suspect him, probably because that he's rich. And I really got obsessed whenever I hear the phrase "50 years ago" or "half a century ago"...
• Atsushi Miyano: Less suspicious than the other twos but there's still a possibility that he's the Boss. His death remains as a mystery, and unlike his wife, he was never showed in canon (despite having been mentioned once or twice) for some unknown reasons. Not much is known about his personality. Also, Haibara's strange reactions point out that she might know something that we didn't.
• Elena Miyano: Just like her husband, however, her face was actually drawn (though not clearly). Despite having been described briefly, her actions and her personality do not make her less mysterious than her husband.
• Someone from the Suzuki family (but not Sonoko nor Jirokichi): Not really suspicious, but they're rich and have many connections.