A
The following Pokémon cannot be obtained in XY without the use of PokéBank:
Rattata, Raticate
Meowth, Persian
Koffing, Weezing
Porygon, Porygon2, Porygon-Z
Mew (not needed for completing National Pokédex)
Chikorita, Bayleef, Meganium
Cyndaquil, Quilava, Typhlosion
Totodile, Croconaw, Feraligatr
Misdreavus, Mismagius
Unown
Stantler
Raikou, Entei, Suicune
Lugia
Ho-Oh
Celebi (not needed for completing National Pokédex; obtainable by registering with PokéBank before a certain date)
Treecko, Grovyle, Sceptile
Mudkip, Marshtomp, Swampert
Torchic, Blaziken, Combusken (available through a now-ended WiFi distribution for XY)
Slakoth, Vigoroth, Slaking
Baltoy, Claydol
Feebas, Milotic
Castform
Regirock, Regice, Registeel
Latias, Latios
Kyogre, Groudon, Rayquaza
Jirachi (not needed for completing National Pokédex)
Deoxys (not needed for completing National Pokédex)
Turtwig, Grotle, Torterra
Chimchar, Monferno, Infernape
Piplup, Prinplup, Empoleon
Kricketot, Kricketune
Cherubi, Cherrim
Buneary, Lopunny
Glameow, Purugly
Finneon, Lumineon
Uxie, Mesprit, Azelf
Dialga, Palkia, Giratina
Heatran
Regigigas
Cresselia
Phione, Manaphy (Manaphy isn't needed for completing the National Pokédex, but Phione is)
Darkrai (not needed for completing National Pokédex)
Shaymin (not needed for completing National Pokédex)
Arceus (not needed for completing National Pokédex)
Victini (not needed for completing National Pokédex)
Snivy, Servine, Serperior
Tepig, Pignite, Emboar
Oshawott, Dewott, Samurott
Cottonee, Whimsicott
Darumaka, Darmanitan
Yamask, Cofagrigus
Frillish, Jellicent
Tynamo, Eelektrik, Eelektross
Elgyem, Beheeyem
Bouffalant
Cobalion, Terrakion, Virizion
Tornadus, Thundurus, Landorus
Reshiram, Zekrom, Kyurem
Keldeo (not needed for completing National Pokédex)
Meloetta (not needed for completing National Pokédex)
Genesect (not needed for completing National Pokédex)