Regigigas

Yet another sad case of a good pokemon given a mediocre moveset. Before it was added to the game master, the Pokemon Go community was hoping to see the trio-master of the legendary titans as a potent normal-type generalist or possibly a pseudo-fighting specialist, wielding its good bulk and titanic attack to great effect in both regards. Instead, Niantic opted to follow theme above effectiveness, giving the Colossal pokemon charged moves with types that used to reflect the trio that it lords over. Regigigas' huge attack does allow it to function as a somewhat serviceable attacker in some cases, but its lack of synergy between its charged moves and quick moves leaves it in an awkward and ineffective position, as much like its trio, it's outclassed in all three of its main types by non-legendary Pokemon.

It's worth noting that in the main series, Regigigas has the normal-type signature move Crush Grip, so it may see some changes in the future. But the move itself has to be obscenely powerful to leave any kind of impact. Even then, Regigigas's best use will probably remain as a partly cloudy generalist.

Best PvE Offensive Moveset

• Hidden Power is more useful than Zen Headbutt as long as a relevant typing can be obtained. 
• Giga Impact is very powerful with STAB, and will outperform other charge moves as long as it's not resisted (not factoring weather). Return* can work as well, for those who purified Regigigas.
• Focus Blast offers wide coverage, and could synergize with certain Hidden Power typings, but maybe consider an actual Fighting-type Pokemon instead. 
• Thunder is an inferior option, consider an Electric-type Pokemon instead. 

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. 

Thunder is the primary charge move due to its lower energy cost, which generally sees the most use due to poor energy generation. Return, exclusive to purified Regigigas, is its next cheapest option and deletes just about any Pokemon that doesn't resist it. Focus Blast is recommended over Giga Impact despite the later benefiting from STAB, primarily due to its coverage.