That said Corey,
I don't know just which one but one in the close to straight walled cartridges would have to be it.
For example, a 30-30 with a 170 grain bullet is not as fast as a 32 special with the same weight of powder and a 170 grain bullet.
A 30-06 and 150 is faster than a 270 and 150 bullet but the 270 is flatter trajectorywise because of the aerodynamics of the skinnier bullet.
The length of the bullet is a big factor. The shorter bearing surface on the bullet is the path of least resistance. If someone could build a round that was perfectly straight it would win.
The 22LR is a lead bullet and has very little drag. The same is for other rounds loaded up with swaged lead bullets.