Is regigigas stronger than Dialga

Answer 1

Let's do this by base stats to avoid the problem of determining a moveset. I am taking Regigigas' ability Slow Start into account.

HP: Even, both have 100.

Attack: Dialga has 120; Regigigas has 55 (after Slow Start; 160 normally.)

Defense: Dialga has 120; Regigigas has 110.

Special Attack: Dialga has 150; Regigigas has 80.

Special Defense: Regigigas has 110; Dialga has 100.

Speed: Dialga has 90; Regigigas has approximately 35-45 (after Slow Start; 100 normally.)

I didn't bother to calculate Regigigas' speed exactly because it's fairly obvious that Dialga is faster. With 4 stats higher and 1 stat tied with Regigigas, Dialga is clearly better - and Regigigas' 10 point special defense difference isn't likely to have any effect.