I'm not sure but I'm going to have to say feebas because its hard to find and some say its rare that's why its hard to catch
If the Pokemon is extremely uncommon, then it is rare.
Rock Tunnel (Uncommon) and Route 10 (Uncommon).
Aipom can be found by using Headbutt on trees in mountain areas. It's one of the most uncommon Pokemon you can find by doing so.
Rock tunnel Uncommon
All the Sinnoh pokemon exept Dusknoir and Spheal and uncommon ones like that.
Purple is the most uncommon eye color
Route 38,39,47. their uncommon
By doing the researching of colors, i think the most uncommon color is Razzmataz
Drowsee is uncommon in route 11.
Meteor Falls. (Uncommon appear rate)
Yes, in the Ruins of Alph(uncommon)
Uncommon catch from meteor falls