Dragonite can serve you well. Its STAB attacks can safely take down Erika and part of Red's team, and it resists Misty, Blaine, and Giovanni.
Ho-Oh is also very good. It has a massive BST, and its STAB moves take out Erika and part of Red's team nicely, while resisting Blaine and Giovanni.
Either works, but as you said you wanted a Dragon/Flying type, go for Dragonite.
Hope I helped!