Wiktionary
tool of choice
n. An object or method (especially software) chosen to solve a particular problem as being the most appropriate instrument.
Usage examples of "tool of choice".
For all they knew, it might also have been the tool of choice utilized to dislodge the significant growth of unidentifiable greenish material from his teeth, which gleamed more or less whitely as he smiled at his saviors.
Most of the time she was content to use information as her tool of choice and rumor as her means of delivery.