ruby/sapphire/emerald

Mew can only be found once on faraway/world edge island and only on pokémon emerald. You'll need an old sea chart to go to it(Nintendo event)

Celebi can only be received from a bonus disk given away with the Japanese pokémon Colosseum

Jirachi can be received from a bonus disk away with the Japanese pokémon Colosseum or can be obtained from pokémon Chanel when played out

Chimecho can only be found on the top of Mt. pyre, it is very rare but if you walk around long enough you could find him

if you use the EON-ticket (Nintendo event) on pokémon ruby you should get to southern island and find Latias, you should be able to find her in Pokemon sapphire or emerald (chose red) after you've beaten the elite four. you'll see a report on TV and she will run around all over hoenn (once you saw her you could see her on you pokédex track her down(don't fly to where she is))

oldies:

red/blue/yellow/gold/silver/crystal

I've got Mew on my Cristal that I've obtained on my yellow trough a Nintendo event. to know the cheats: click here

the sticker they've put on my yellow for getting mew

Celebi is only obtainable in Nintendo events. I've missed one :'(