Without Cheat codes, Action Replay etc
Just to tell you, not all Pokemon events happen in all the countries. For example, Meloetta is only available as an event in Japan.
This will tell you everything
All the legendaries that are event based are mew, celebi, jirachi, deoxys, manaphy, phione, darkrai, shaymin, arceus, victini, keldeo, meloetta, and genesect.