In Pokemon lore, Regigigas is the one who was powerful enough to create the three other titans. Though Regigigas created Regice, Registeel, and Regirock, the Pokemon tends to fall short in Pokemon GO. Much of that has to do with the moveset, but players can at least attempt a good one.
When choosing a Fast Attack on Regigigas, the best option is Hidden Power. The move appears as a Normal-type but is a Rock-type move on the Pokemon. Hidden Power will have the same attack type bonus as well as having the highest dps and the highest energy per second gained on Regigigas.
For the Charge Attack, players should choose Thunder as their first option. The Electric-Type move has the fastest charge time and still packs a significant punch. Plus, Thunder is a decent option in the Ultra League or higher.
If players get enough Regigigas candy, they can get a second Charge Attack, which should be Focus Blast. Giga Impact is the only other option, and Focus Blast will serve as a more useful nuke attack for less energy.
Is Regigigas a good choice in Pokemon GO PVP battles?
The short answer for this legendary titan is simply no. Regigigas falls flat in Pokemon GO PVP even though it may have some bulky base stats like the other titans.
Unfortunately, the moves that Regigigas has access to just don't cut it. They are weak and don't charge enough compared to half the other moves that are available in the Ultra League.