!DUNSPARCE IS JUST AN EXAMPLE, FOLKS! Because I totally disagree, Dunsparce SHOULD evolve...anyway. What Pokémon do you think shouldn't receive an evolution? In my opinion, Casform should not. Castform is a manmade Pokemon, and is gimmicky. Giving it an evolution would benefit it, sure, but I don't think it'd keep its gimmick upon evolution, which would kill the purpose of Castform as a Pokémon.
Other than that, no other Pokémon come to mind. By the way, this question can be for Pokémon that don't evolve, or Pokémon in a two-stage evolutionary line.
Seviper and Zangoose are both pretty middle of the road Pokemon in terms of fighting ability and stats if you look towards the games. With their story as natural born enemies though, I don't see an evolution or mega evolution making sense for either of them, and while they're not great on their own, I'd be happy to leave them just the way they are.