Project Pokemon Wiki:Pokémon Rarity Guide

Trading Values
Pokémon values are very confusing, some legendaries are less rare than normal Pokémon, some shiny legendaries are rarer than others, it's a lot to take in. The trading value guide below will help you understand it a bit better.
 * Rayquaza = 2 Kyurems OR 2 Giratinas OR 2 Palkias OR 2 Dialgas
 * Kyurem, Palkia, Dialga, Giratina = Groudon AND Kyogre
 * Groudon, Kyogre = Suicune, Entei, Raikou
 * Mewtwo, Darkrai = 2 Fully EV & IV trained Pseudo-Legendary OR Latios AND Latias

Rarest Pokémon/Hardest to Obtain (Greatest to Least)
!!!THE LIST STARTS HERE IF YOU DON'T INCLUDE THE ROULETTE, IMPOSSIBLE, AND EVENT POKÉMON!!!
 * 1) Arceus (impossible to obtain in the original, only Mod+ could obtain it.
 * 2) Rotom (passed event, otherwise impossible to obtain)
 * 3) Jirachi (ultra-rare, roulette only)
 * 4) Diancie (very rare, roulette only)
 * 5) Zygarde (very rare, roulette only)
 * 6) Victini (very rare, roulette only)
 * 7) Phione (very rare, roulette only)
 * 8) Genesect (very rare, roulette only)
 * 9) Hoopa (very rare, roulette only)
 * 10) Azelf, Mespirit, Uxie (very rare, roulette only)
 * 11) Tornadus, Landorus, Thundrus (very rare, roulette only)
 * 1) Rayquaza (hunches from the NPC in Cinnabar Volcano, changes within 10 minutes, 1/3 chance with other 2 legendaries/mythical) Rarity Level = Exotic
 * 2) Reshiram (must have a fire type in your party, must not have an electric type in your party, find inside of Mausoleum of Origins.) Rarity Level = Uber Rare
 * 3) Zekrom (must have an electric type in your party, must not have a fire type in your party,  find inside of Mausoleum of Origins.) Rarity Level = Ultra Rare
 * 4) Giratina (must have the Champion Badge, find at the Pokémon Tower, hard to find and capture because you can't use False Swipe) Rarity Level = Super Rare
 * 5) Kyurem (must have the Champion Badge, find at Seafoam Cave) Rarity Level = Super Rare
 * 6) Dialga, Palkia (must have the Champion Badge, find Dialga at Victory Road, find Palkia at the attack status area in Elegant Valley) Rarity Level = Super Rare
 * 7) Groudon, Kyogre (must have Rayquaza in your party while looking for it) Rarity Level = Very Rare
 * 8) Deoxys (must have Solrock and Lunatone in your party, must search in the Pichu nest where you climb up the ladder and go across the bridge) Rarity Level = Rare
 * 9) Keldeo (must have the Champion Badge, find at Route 12) Rarity Level = Rare
 * 10) Lugia, Ho-Oh (find Lugia at the Inside of Mausoleum of Origins, find Ho-Oh at Victory Road) Rarity Level = Rare
 * 11) Regice, Regirock, Registeel (find Regice at Cinnabar Volcano, find Regirock at Mt. Moon, find Registeel at Victory Road) Rarity Level = Rare
 * 12) Cobalion, Terrakion, Virizion (must have Keldeo in your party, you can find them anywhere) Rarity Level = High
 * 13) Zapdos, Moltres, Articuno (find Zapdos at Route 10, find Moltres in the orange patches of grass at Cinnabar Volcano, find Articuno inside of Seafoam Cave) Rarity Level = High
 * 14) Cresselia (hunches from the NPC in Cinnabar Volcano) Rarity Level = Medium-High
 * 15) Manaphy (hunches from the NPC in Cinnabar Volcano) Rarity Level = Medium-High
 * 16) Darkrai (you can find it anywhere, must be night time) Rarity Level = Medium
 * 17) Mewtwo (you can find it anywhere) Rarity Level = Medium
 * 18) Shaymin (random swarm) Rarity Level = Medium
 * 19) Heatran (talk to Professor Oak in Saffron City with 300+ Pokedex entries to obtain it) Rarity Level = Medium
 * 20) Celebi (you can find it anywhere)  Rarity Level = Medium-Low
 * 21) Suicune, Raikou, Entei (you can find it anywhere or talk to the NPC in Viridian City's Pokémart to obtain one randomly, you must have the 'Other Legendaries' gamepass for it though) Rarity Level = Low
 * 22) Latias, Latios (you can find them anywhere)
 * 23) Mew (impossible to find without gamepass, extremely common with it though, to get one for free, talk to the NPC inside of the Poké Center, you must have the 'Mew' gamepass though) Rarity Level = Low
 * 24) Meloetta (finding all the music notes at the Mausoleum of Origins, then talking to the NPC inside of one of the houses at the Mausoleum of Origins) Rarity Level = Low

Demand (Greatest to Least)
If there is a Pokémon that is not on this list, the demand wasn't high enough for it to make it on.
 * 1) Rayquaza - Extremely High Demand
 * 2) Mewtwo - Extremely High Demand
 * 3) Kyurem - Very High Demand
 * 4) Giratina -  High Demand
 * 5) Darkrai - High Demand
 * 6) Dialga - High Demand
 * 7) Palkia - Medium-High Demand
 * 8) Groudon - Medium-High Demand
 * 9) Suicune - Medium Demand
 * 10) Entei - Medium Demand