There are a ton of alien species some people believe visit Earth, but these are the most popular.
The Reptilians, the Grays, and the Nordics.
I could talk forever about this, like the reptilians are usually considered the original inhabitants of Earth but are always lumped in with aliens. Also, they were invented by an American science fiction writer but were made popular in modern alien lore by an Englishman.
The Grays are an all American invention which is also why we see them most in fiction. The Americans make the most alien movies and shows.
Finally there's the Nordics, not to be confused with the actual Nordic people. They are called Nordics because they look like Nordic stereotypes and because they were originally only reported in the Nordic countries. They have since been replaced in Nordic UFO rapports with the Grays.
I used Norway to represent the Nordics here because he is pretty damn in love with himself and it's so rare that I get to show that part of his personality.