Rarest Pokemon Ever?

[Morningstar]

Right now I'd say Mew, Arceus, Diancie, and the Unova event pokemon (except maybe Victini) goinly solely by attainability. (all of these are event only pokemon and has at most 2 events to this day)

EDIT: forgot Celebi

 

[Absent]

Manaphy and Jirachi. I got lucky by downloading the Manaphy mission before Ranger Net became obsolete.

 

[sargeantsimon]

Ooh, this reminds me of the time I calculated the odds of finding the rarest thing in Pokemon, aside from anything requiring a specific Spinda pattern for obvious reasons. As it so happens... it's Unown. A shiny, 6IV Unown F with a specific nature, to be exact. I can't remember all the numbers, but after all the calculating, I ended up with hundreds of thousands of years of doing nothing but forcing Unown encounters to get just one.

I was more than a little tempted to hack one in, but I resisted.