Spock does have a slightly off hue, you can tell whenever he is next to other characters. It was supposed to be green, but is pale so hard to tell, but it isn’t pinkish like caucasians.
On Next Gen and on, Romulans and Vulcans seem more grayish, except of course for Tuvok.
Kang and Kodos wouldn’t rank in the top 50.
Sure they would, if for no other reason than I doubt most people can name 50 aliens.
I thought you were going to post the German speaking ones 
Superman. Luke is human the way Paul Atreides is (or starts as) human. It is an essential part of Superman’s story that he is not human.
A challenge! Let’s see, including only individual aliens (not groups or types of aliens) in pop culture, and working with no references and strictly from memory…
Superman
Spock
Yoda
E.T.
Marvin the Martian
Kang
Kodos
My Favorite Martian
The Great Gazoo
J’onn J’onzz, The Martian Manhunter
The Silver Surfer
Galactus
Klaatu
Tars Tarkas
The Blob
Ming the Merciless
Worf
Sarek
Dr. Zoidberg
Kiff
Supergirl
Krypto
Jor-El
Lara (Superman’s mother)
Predator (not sure if that counts since the character doesn’t have a name)
…
…and I’m spent at 25.
Alien(s)?
The correct answer is Michael Jackson.
Europe doesn’t have that shape though? But I suppose you’re right.
This would be a great Family Feud question. “We surveyed 100 people and asked them to name a famous alien…”
I think E.T. would be very close with Yoda, at least for the over 30 set, if only because E.T.'s actual name indicates his alienness.
I don’t think Superman would rank as highly as those two in such a survey, mainly 'cause I don’t think he springs to mind as an “alien,” exactly.
BUT I do think that Superman is the correct answer to the actual question posed in the OP. Even if people don’t think of him as “alien,” Superman is a) an alien, and b) vastly more popular and well-known than any of the others mentioned, including Spock. IMO. 
Since we’ve already pretty much agreed that humanoids which are indistinguishable from H. sapiens aren’t to be considered. Or at least I think that was the consensus, now I’m not sure. If Superman just had a third eye or cauliflower ears or something, then he should win hands down. If we limit the discussion to beings that can not pass themselves off as human, then I would guess that Chewbacca would be the alien that the largest percentage of people on Earth could identify from an unmarked photograph.
An interesting way to modify the question is to ask which alien species is most recognizable to the largest number of Earth’s populace. Most people would balk at identifying Chewbacca’s species (I certainly don’t know what it is). If the question were phrased that way, I would guess that the answer would be Klingons.
Superman’s in the lead, but let’s not forget Klaatu.
Interestingly, the same could be said about ALF.
I see the logic in naming Superman the most known alien, but I question that his alien origins are as well known as some posters imply. I think that many who know him know him as a caped superhero foremost and not necessarily that he comes from another planet.
In the case of Yoda, you don’t really have to know anything about him to see that he’s alien. That would give him an upperhand on Superman.
…Gort
The badass from Predator (very useful if we’re going to have ALF around.)
Jar-Jar (see note above)
My favorite Martian. (Maybe just the over forty set?)
Gazoo?
I thought of this earlier… how do you fill out the top ten? Even if they aren’t cultural icons?
(I would think Aliens would only count as one. Likewise with Jar-Jar.)
Optimus Prime.
Isn’t alive.
As a matter of fact, unlike Mr. Spock, the others aren’t even real.:rolleyes:
It’s those damned balrogs. Moving things around.
My first thought also.
Spock’s thought processes are alien. Superman’s do not seem to be.
Gazoo?
Is real!
Heathen!
I brought up Spock’s imposture as an Earthman myself, but I feel obliged to point out that he was mostly staying in the flophouse room he & Kirk rented, as Kirk was supporting them financially for the most part (to give Spock the time to work on his tricorder). If Spock had been alone, I don’t think he could have kept it up for long. His instincts are all wrong. He doesn’t laugh. And I wouldn’t be surprised if his body language is very stiff. He’s going to make humans who don’t realize hwhat is is suspicious and antsy on a subliminal level.