Not to mention the Gen 8 DLC that created 2 more Regies to add to the already existing 3. :p
Well I would say the Legendary beasts from Gen 2, but because those less than stellar looking 'primal' versions now exist thanks to Gen 9, I don't want to risk making them worse... >.>
So I'll go with Ho-oh and Luigi Lugia. It's always seemed odd to me that Gen 2 didn't have the more common 3 Legendaries from other games, (not including Gen 1) so giving them and 'actual' '''Crystal''' (I'm sure someone's thinking 'Suicune!', but no, that's one of the beasts, it can't also be a 'major Legendary' imo) Pokemon would complete the group.