Uncle Martin (My Favorite Martian)
Not a conqueror but an anthropologist, Exigius 12 1/2 crash-landed on Earth while studying mankind and was taken in by a reporter while he repaired his ship. He hid his vast array of mental and technological powers from other humans, but he could likely have enslaved us all single-handedly. Instead, he mostly transported historical figures through time and turned squirrels into people.

Gordon "ALF" Shumway (ALF)
One of the few survivors of the nuclear destruction of the planet Melmac, ALF also crash-landed on Earth, and although he was part of the planet's space fleet, he mostly just hung out with a human family as he repaired his ship, helped the kids with their homework and tried to eat their cat.

Evie (Out of this World)
When Antarean Troy Garland crashed on Earth, the only thing getting invaded was human Donna Froelich's pants. After marrying her and mixing his alien DNA with hers, he was called back to Antareus to fight in a war, leaving Donna to raise their super-powered daughter Evie. We only follow Evie's story until her 18th birthday, but we'll assume she didn't use her teleportation power, time-freezing power, and the ability to create objects with her mind to take over the world.

Sam George Francisco (Alien Nation)
While the Tenctonese "Newcomers" successfully invaded human society, they did it openly, as immigrants, and gradually assimilated into society by taking jobs alongside humans. The fortunately re-named Francisco became a cop to put sour milk in the fridge and beaver on the table.

Teal'c (Stargate SG-1)
A Jaffa from the planet Chulak, Teal'c defected from the Goa'uld System Lord Apophis to the human SG-1 team, and although he lived on Earth, his work took him to hundreds of planets across the galaxy. Still, he helped defeat the evil Goa'uld, who used humans as host bodies for their parasitic forms.

Max, Isabel and Michael (Roswell)
Human-hybrid clones of the king of Antar, his sister, and his second-in-command, these three crash-landed in Roswell, New Mexico ("Area 51") in 1947, and yet are still attending high school there in 1999. If this is an invasion, it's the Worst. Invasion. Ever.

Dick Solomon (3rd Rock From the Sun)
The commander of an alien expedition sent to infiltrate and study human society, Dick is actually the youngest and most immature of the four team members, which makes his older, assigned human form unfortunate. He earns money as a physics professor at the university, and even dates a human woman, who obviously is not very picky.

The Doctor (Doctor Who)
He's been visiting Earth in various guises since the 1960s, and has hopped around time from the Elizabethan era to the distant future, but he's saved his adopted planet more times than anyone can count, from a variety of invading alien nasties.

Dr. Zoidberg (Futurama)
A doctor sworn to help the sick and injured, Zoidberg is possibly more dangerous than helpful, since he actually knows very little about human anatomy. And while his kind have lived peacefully on Earth among humans and other aliens for years, it's Zoidberg's eating of the Earth flag that inspires the Decapodian race to invade the planet and save him from being executed. So technically, he started a war between worlds, but it's not like it was his idea.

John Jones (Smallville)
Yeah, yeah, we know that Clark Kent is an alien visitor who's been here since infancy, but what has he ever done for Earth? He's put it in danger more times than we can count. But J'onn J'onzz, the Manhunter from Mars, is actually contributing to human society as a police officer, and has bailed Clark out on more than one occasion. And now that he's got his super powers back, Clark is totally redundant.

Who's your favorite alien visitor on TV? (Don't say Mork. Don't say Mork. Don't say Mork.)