Most effective alien infiltrators?

[Darmalus]

The Yeerks from the Animorph books were pretty decent infiltrators. Once in your head, they had total access to your memories, and could replicate the normal behavior well enough that no one would notice any change. The only limit they seemed to have was they had to leave your head and soak in a pool of nutrients every so often.

I think that the Thing would be the best, it could replicate anyone, reproduce frighteningly fast, and seemed highly intelligent.

[Swindle1984]

Watching the TNG episode 'Conspiracy' lately,.

Is that the one where they get brainwashed by some computer game?

It was one with the purple maggots taking over Starfleet Admirals. You know, the gory one with the quasi-chestburster thing at the end.

I was three or four when that episode came out and it freaked me the fuck out when their phasers blew that guy up and a monster was in his gory chest cavity.

[Nathaniel]

nBSG Cylons were pretty effective infiltrators. Their almost complete victory over the 12 colonies was largely a result of how convincing they were, at least to Baltar's dick.

[Metahive]

The Thing from the movie of the same name. Bonus points for being able of assimilating people covertly. I think its only weakness is going atavistic once its fight or flight reflex is triggered.
Then there are also of course the Pod People of Body Snatchers fame. The only imperfection they have is the lack of emotion the copies suffer from, compelling people to instinctively mistrust them, otherwise they'd be probably the most effective.

[Talhe]

The advanced Face Dancers from Dune. Virtually impossible to detect and able to assimilate memories makes them nearly undetectable.

[Metahive]

However, they run the risk of seriously forgetting they're infiltrators in the first place and fully becoming what they copied. The versions that just take small memory imprints are better.

[The Yosemite Bear]

Mice and Dolphins failing that housecats.

as per descriptions from Hitchhiker's Guide for the former two, and HP Lovecraft for the later.

I mean whose going to notice a godbieng when it's at your door meowing, fucking with your behaavioural test just to see your reaction, doing really perverted stuff, eating fishh, and not worring about what all those humans do.

[PeZook]

Aliens from "V" weren't really infiltrators. Nothing would change if they didn't wear the fake skin ; They could do the exact same thing they did in the series (coming in hugeass starships, offering us their tech in exchange for "important chemicals" and then stealing all our precious bodily fluids )

Although, frankly, if they stayed as EVIL REPTILLIOIDS it would've actually made them more likely to succeed, because their incompetent security people would be able to tell human infiltrators from actual Visitors way more easily. So they shouldn't count ,because their lameass attempts at "infiltration" were actually detrimental to their overall goal due to sheer incompetence (seriously, a guy with no disguise, no documents and hauling a huge 1980s camcorder snuck in and out of a mothership by the brilliant strategem of "jump onto a shuttle before the door closes" Man...) Of course, the show's budget would suffer, but hey, SoD and shit...

[Lord of the Abyss]

Mice and Dolphins failing that housecats.

as per descriptions from Hitchhiker's Guide for the former two, and HP Lovecraft for the later.

I mean whose going to notice a godbieng when it's at your door meowing, fucking with your behaavioural test just to see your reaction, doing really perverted stuff, eating fishh, and not worring about what all those humans do.

Ah, that reminds me of the old story Into Your Tent I'll Creep. In it a guy is explaining to his friend his theory that dogs are sentient, that they are faking being stupid so they can live off us; the listener scoffs of course. Then as they are leaving down some stairs, suddenly a dog runs out of nowhere, trips the conspiracy guy so he falls and breaks his neck. Then the dog pauses just a moment, looks the friend straight in the eye, and then turns and runs off.

[TOSDOC]

The Puppet Masters of Heinlein's novel were VERY close to catching the President, and the whole country had to strip naked just to tell friend from foe. That's some pretty drastic social implications for combatting a covert alien invader.

Check out the movie The Hidden--if that alien's personality wasn't so vain, he could have had a seat of power much sooner than the movie depicted, and the story could have taken a completely different turn.

The Thing had the best canine actor I've ever seen in a movie--it still creeps me out the way the Husky stares at the men or out the window, creeping around looking for another man to take over. I think of him every time a husky walks into my practice.

[espatier]

I'm going to take a stab at this one and say..... Mexicans that cross the border into the U.S. and Canada.