It is easy. Leaf Tornado is way weaker than Energy Ball. So choose Energy Ball.
Serperior uses Leaf Blade more effectively than Energy Ball because of the stat boosting move Coil.
commented 8 minutes ago by ~TastyMushroom~ ):<
TastyMushroom is correct - Since Serperior naturally learns Coil which bolsters her awful Atk stat, go with Leaf Blade. Energy Ball however, Is also an option if you want a Special-oriented Serperior. It won't really make any difference since Serperior's SpA and Atk stat is the same 75 each.
Serperior @ Leftovers/Anything
- Energy Ball/Leaf Blade
- Coil/Calm Mind
However, If you can get Contrary Serperior then use this set instead:
Serperior @ Big Root/Wide Lens/Leftovers
- Leaf Storm
- Giga Drain
(It's pretty unlikely to get Contrary though, as you can only obtain it via Dream World or Breeding, but Contrary + Leaf Storm is awesome!)
Hope I helped!