Rayquaza= best offensive dragon in raids << replacable by dragonite
Palkia= best overall dragon by CP, and theoretical power. Falls slightly short due to one bar move limitation in practice. << replacable by salamence but the gap is wider than dragonite v rayquaza
Latios= best balanced dragon for master league PVP/Raid utility
If your needs are for raids, power up either Rayquaza or Palkia. Do Latios if you want PVP power and still a respectable raid attacker.