First of all, you can catch all the non-legendairy pokemon. You can catch all the Hoenn legendairies. Mew is unobtainable until Emerald comes out, and no one knows how to get Celebi. Ho-oh can be caught in coloseum, or in LG/FR with a special ticket, and Lugea can be caught with that ticket. Mewtwo, Articuno and Zapdos are in the same places they were in RBY, and Moltress is on One Island. Suicune, Entei and Raikou are all obtainable in Coloseum, and LG/FR. This leaves us with two unobtainable pokemon:Sephiroth said:How many pokemon can yopu get without cheating? (Including Lugia, Ho-oh and Deoxeys).
Toothache said:Lugia - Mystic Ticket
Ho-oh - Mystic Ticket, or Colosseum
Deoxys - Aurora Ticket
Jirachi - Colosseum bonus disc (US), or Pokemon Channel (UK/Europe/Australia)