Reuniclus

Poor Reuniclus has a severe lack of niche. As a Psychic Type attacker, it's in direct competition with Mewtwo and Shadow Mewtwo, which is like comparing a zippo lighter to a burning building. It's passible on defense, but still fairly easy to counter. Sadly, this Pokemon is little more than a dex filler.

Best PvE Offensive Moveset

• Zen Headbutt and Future Sight give Reuniclus its strongest offensive moveset.
• Hidden Power is inferior, even if it's Psychic type.
• Shadow Ball and Thunder both fall short.

Best PvE Defensive Moveset

• Hidden Power is the defensive fast move of choice, especially if it has a type advantage over Reuniclus' counters in Dark, Ghost, Steel, and other Psychic Types.
• Zen Headbutt isn't bad, but it's generally inferior.
• Shadow Ball has high power, great neutral coverage, and comes out frequently
• Thunder is the move of choice when defending against most Dark types, but it takes a lot of energy to launch.
• Future Sight has very few advantages for Reuniclus on defense, and should be TMed.

Hidden Power is superior to Zen Headbutt, and can also surprise opponents with its unpredictable typing. 

Shadow Ball costs the least amount of energy, deals considerable damage, and provides coverage against Ghost-types, in addition to relatively neutral coverage. Thunder provides additional coverage and costs slightly less energy than Future Sight, while the latter benefits from STAB.