In your opinion, what's the rarest/most difficult to obtain Pokemon?
RSE Feebas, no contest. Game Freak made the little guy much, much easier to find in ORAS, but it was quite the gem in the originals. Out of all the water tiles at Route 119, Feebas can only be fished up from 6 randomly determined tiles on the map; this was randomly re-shuffled whenever the trendy phrase at Dewford Town was changed. I used to spend dozens of hours worth of play time to find a tile, let alone catching an optimal nature for evolving it. I filled up boxes of them as soon as I found and remembered a spot; because of their rarity, you were able to get pretty much any Pokemon you wanted from trading with other players (unless they've found their own Feebas tile)