Not sure why you are limiting yourself to one lift?
I'm no expert on lifting but I don't believe there is a "best" lift. You may be better served by seeking the best lifting/conditioning program.
If I were limited to doing one lift/exercise/drill to increase speed, I would be on a long toss program. But again, why limit yourself?
Also, there is a whole forum on this board devoted to your question called Strength/Conditioning. Yours is such a general but important question that if you used the search function, you will find your question has been answered many times and in many ways over the years.
Just trying to help. Good Luck.