Post game meaning after defeating the E4, not after catching Groudon or Kyogre. Also, if any Pokemon on the list can also be found in X and Y, then don't include it.
After rematching the elite four
Chikorita, Cyndaquil, or Totodile,
Snivy, Tepig, or Oshawott,
Turtwig, Chimchar, or Piplup
And then rayquaza and deoxys
(Sorry about how I wrote it)
sumwun's edit: There's also Beldum.