I have found Agility to be the best set for me:
Diggersby @ Life Orb
Set: Agility baby guurrrrrrrrrrrl
Ability: Huge Power
Move 1: Return
Move 2: Earthquake
Move 3: Stone Edge
Move 4: Agility
Nature: Adamant
EVS: 0/252/0/0/4/252
With one Agility, Diggersby hits 510 speed - outspeeding...