Easy. 10mm. Learn to reload. With the proper and inexpensive conversion parts, that one gun will also shoot 50 GI, .40 S&W, 9x25 Dillon, 357 SIG, .22LR.
You haven't lived until you shoot 9x25 Dillon at it's max.
With a stock G20, you can shoot .40 S&W all day.