Giratina > Palkia > Dialga. Giratina had a secondary forme plus its own unique dimension unlike the other two dragons.
Well Xerneas and Yveltal got 1 game each. Zygarde sort of got 6 since it got its Formes and other signature moves in Alola and was given a bit more importance as a side quest in SM.This is very good trio. But with all the respect, they have had enough love during they years and 6 games.
The xy Legendary trio got......... 1 game.