Don't know if it's the correct place to write it but here my top 10 rarest Pokemon to catch during normal gameplay (I'll add special gameplay later):
-N°10 Stakataka in UM and Blacephalon in US (only 2 of both in one version each, quite rare but there's so much rarer)
-N°8 Skuntank in Diamond (whereas Purugly can be found in ORAS and Pearl Skuntank remains exclusive to Diamond)
-N°7 Hitmontop in Colosseum Hitmonlee/Hitmonchan in XD (only in one example, very hard to catch, it's the only way to shinyhunt Hitmontop)
-N°4 Togetic in Colosseum (only one at the end of the game, only way to shinyhunt it) or in B2W2 during event Funfiesta mission in Japan
-N°3 Celebi in Crystal (VC or Japanese Cartridge during an online event, only one, only way to shinyhunt it legally)
-N°2 Mew in Emerald (legally only in Japan during an event, only way to shinyhunt it legally)
-N°1 Scizor in Colosseum (only one, in Japan with e-reader cards, honnestly which one of you really caught it?)
Special gameplay:
-Old Man glitch in Gen1 Venusaur, Charizard, Aerodactyl, Alakazam (cannot be shinyhunted)
-Emerald Battle Pyramid : Villeplume, Jolteon, Ludicolo, Shedinja, Flygon, Shiftry, Exeggutor, Alakazam, Omastar, Armaldo, Cradily, Aerodactyl, Machamp, Flareon, Blastoise, Charizard, Arcanine, Vaporeon, Porygon2 (can be shinyhunting by glitching)
-XY demo: Exeggutor