Porygon is not the most exciting on the list, but it's not easy to find one.
3. Venusaur, Blastoise, Charizard
It's kind of cheating with three ranking third, but it actually makes sense to lump these next three groups together rather than randomly sorting the Pokémon in those groups into arbitrary rankings.
These are all rare and they don't hang around on your nearby list for you to finish up what you're doing should one happen to be close to you. It's easy enough to find a Bulbasaur, Squirtle, or Charmander to start the evolutions, but it takes a lot of time to make it work.
2. Tauros, Farfetchd, Kangaskhan, Mr. Mime
These are the region-specific Pokémon. Tauros can only be found in the wild in North America, Kangaskhan in Australia and the Pacific Islands, Mr. Mime in Europe, and Farfetchd in Asia. They can be hatched from 5km eggs, but with so many Pokémon in the 5km category, it will take some work.
1. Mew, Mewtwo, Articuno, Zapdos, Moltres, Ditto
These "legendaries" (except Ditto) are, according to Ali, yet to be found by any user on Pokémon Go, but could be coming soon.
This list should cause no shortage of people with some differing opinions. Maybe Aerodactyl should be here, or maybe you were one of the people losing your god damn mind over a Vaporeon in Central Park. No matter what, if you've got a handful of these you should, ahem, count yourself lucky.