Mine is my Minolta AF 50mm f3.5 Macro. I love this lens for copying slides and general photography. It is wicked sharp, no matter the f stop. A close contender is my 50mm Pentax Takumar f1.4 SMC which I use in low light and portrait work.
I have been very impressed with the Pentax 55 F2 SMC Takumar and the Minolta 58mm f1.2 MC !