Mew was the final original Pokémon in National Pokédex order, listed as number 0151.
The first-generation games formally introduced 151 species, ending with Mew after Mewtwo at number 0150. Mew is a Mythical Pokémon associated with the idea that it contains the genetic makeup of every Pokémon. Its rarity and hidden status made it especially mysterious to early players.
Mew was not normally obtainable through ordinary gameplay in the original releases. It was distributed through special events, which helped fuel rumors and experimentation among players. Mewtwo, despite appearing later in the Pokédex, is not the final entry; it is numbered 0150 and is encountered as a powerful endgame Pokémon.
MissingNo. is also not an official species. It is a glitch-related placeholder encountered through exploits in the Game Boy games. These distinctions explain why Mew is the correct canonical answer rather than Mewtwo or MissingNo.