Regigigas

After years of being unable to get going, Regigigas's slow start finally ends with the release of its signature move, Crush Grip. While the concept of a Normal attacker sounds unappealing due its lack of super effective targets, Regigigas is an incredibly impressive exception. Crush Grip is so immensely powerful that, when combined with Regigigas's titanic Attack stat, it enables the progenitor of the legendary giants to act as a respectable raid generalist. With Partly Cloudy weather and the correct Hidden Power, varying on the boss, Regigigas can often show up amongst the top counters of said boss. Its power only gets more frightening in its Shadow form...

Best PvE Offensive Moveset

• Hidden Power is more useful than Zen Headbutt as long as a relevant typing can be obtained. Zen Headbutt, however, has a shorter duration, which can be more useful in Party Play.
• A notable mention goes to Hidden Power Rock, due to it also being boosted in Partly Cloudy.
• The Elite move Crush Grip is THE reason to run Regigigas. Its parameters surpass some of the game's most powerful Charged Attacks, allowing Regigigas to effectively bypass the type chart with raw power.
• Giga Impact and Return are great moves, but they are incredibly far behind Crush Grip in performance. Return being a 3-bar move is great for Gym clearing, however.
• Focus Blast and Thunder are underwhelming.

Hidden Power is superior to Zen Headbutt, in addition to offering a large variety of coverage options. The specific typing that's more preferable can vary depending on the PvP format, with some typings more relevant than others. Fighting and Ground each cover 5 different typings, including the popular Steel types, Ice covers popular Dragon and Flying types, and Dark is useful against common Psychic and Ghost types in Ultra and Master League. 

Crush Grip is Regigigas's cheapest costing move and hits extremely hard. Thunder is its next cheapest, and can hit Ghost-type Pokemon in the event that its Hidden Power doesn't cover that well. Focus Blast is extremely expensive, but it can delete Steel-type Pokemon. Return and Giga Impact are outclassed by Crush Grip