Ferrothorn, hands down. I admit, there might be others, but it actually has decent Attack, and that God-awful speed stat makes Gyro Ball powered up to insane levels.
Electivire is a good Pokemon for this move.
It has a better Attack than its Special Attack, but here me out:
With its ability, Motor Drive, and general Decent defenses and speed, Electivire can switch in to an Electric move, and then actually use Electro Ball quite well, thanks to its Spee boost.
These are the two that came first to my mind, will edit others later, as I think of them, and do a bit more of research.