The most rarest Pokémon of all, Mew, is in Pokémon Snap. To get him you need to take a picture of six things. They are things that resemble Pokémon but really are not.

Level 1: The Beach
After you pass a Pikachu on the Beach, you will see some mossy rocks. If you pass it and look back at the correct angle, it resembles a Kingler.

Level 2: The Tunnel
There is a pile of junk laying around in the power plant. If you get Zapdos to turn the power on, a light points to a panel on the wall. The shawdow resembles a Pinsir.

Level 3: The Volcano
At the beginning of the level, there is a crater with a Rapidash near it. There is some Purple Gas that resembles a Koffing that comes out of it. To make it appear, throw a pester ball into the crater and the Purple Gas will rise up.

Level 4: The River
There is another mossy rock structure that looks like Cubone. After you play the Poké Flute for the Vileplume that buries itself in the ground, the rock structure will slowly come into view. It resembles a Cubone.

Level 5: The Cave
When you go to Weepinbell's Pool, turn around. You will see something that looks like a constellation of stars on the wall. It resembles a Mewtwo.

Level 6: The Valley
When you enter the Valley's white water, if you look in the horizon, you will see oddly shaped mountains. Take a picture of them and they look like a Dugtrio.

Once you have a picture of all six of these things take them to Professor Oak. He will build a bridge to take you the Rainbow Cloud. There you will find Mew.

How to take a picture of Mew:
When you first see Mew, he will be inside a blue electric shell. He will move around and try to avoid you. You can't take a picture of him when he is in the shell. If you hit the shell directly three times with Pokémon Food or a Pester Ball, Mew will shed the blue shell, He is now in a yellow shell. He is much harder to hit and faster and he will sometimes move off the screen. After you hit the yellow shell three times, Mew will be in the open. You can take a picture of him chasing his shell, but you won't get a lot of points. Hit Mew a few times again with Pokémon Food or a Pester Ball, and he will spin around for a few seconds, then face you for a few seconds. Then after that, he will disappear. When he faces you, Zoom-In to get a good picture.