My Ampharos (in SS) may be my best Pokemon (except Lugia). This moveset I find very effective:
Strength: Decent power, good back-up move, & needed outside of battle.
Discharge: Sufficient power, good accuracy, & 30% chance to paralyze
Signal Beam: Good power, good accuracy, & effective on both Grass & Psychic types.
Power Gem: Good power, good accuracy & effective on both Fire & Ice types.
I know Thunderbolt or Thunder are preferred as an Electric move, but I find Ampharos has high enough Special Attack that it doesn't make much a difference. Plus it seems to always cause paralysis, if it doesn't KO right away.
I kept Flaaffy from evolving until it learns Power Gem, that way Ampharos had it before the Elite Four.