Sheldon seems too black and white for an INTJ (we are often very open to better possibilities). He always refers so much to his past experiences also and how it affected him which is also not INTJ and he can be quite emotional concerning verbal fights... I think ISTJ
INTJs do seem too black and white to outsiders, because they know they are right. And so is Sheldon, from his own wicked perspective. Everything else can be attributed to his OCD (except for the 'emotional verbal fights' part, in which case most INTJs seriously misjudge themselves, methinks).
I think the overwhelming sentiment on Sheldon is that he is an INTJ. I watch TV rather infrequently -though when the TV is on, I could be caught watching Big Bang Theory somewhat regularly. Through the 6-8 episodes I have ever seen, I have tried to type Sheldon , and I anchored upon INTJ after this discourse:
Scene: The entrance lobby. Sheldon is sitting on the bottom step using his laptop. Penny comes down the stairs.
Penny: Sheldon? What are you doing?
Sheldon: Playing Super Mario on a poorly coded Nintendo 64 emulator.
Notice it is not just an "N64 emulator," but a "poorly coded N64 emulator." This conclusiveness and decisiveness in speech patterns is characteristic of xxxJ types.
Penny: Yeah, but why are you doing it on the stairs?
Sheldon: Iím a modern day Napoleon exiled to the Elba of the staircase because Leonard, get this, has a date.
This response hints strongly to perception rooted in possibilities rather than actualities (ie. intuition). Notice he says, "I'm a modern day Napolean..." not "I'm in the staircase like a modern day Napolean..." The former is him taking the form of the possibility (Ni), whereas the later is him applying the objective situation to a possibility (Ne).
Certainly this is hairsplitting and not necessarily completely indicative of the attitude applied to his intuition, but this statement appears more likely to come from an individual using Ni than an individual using Ne.
Penny: Oh. Oh, well, good for him. Yeah but, why are you sitting here, why donít you just go to a movie or something?
Penny: Yeah, why not?
Sheldon: What if I choke on my popcorn, who will administer the Heimlich maneuver?
The very nature of Ni is about foreseeing imperative implications of future events given the current situation. This is a tailor-made example.
Penny: Well then donít order popcorn.
Sheldon: No popcorn at the moÖ listen to yourself.
The tertiary function (ie. "eternal child" or "function of validation") for an INTJ is Fi. This is a great example of Sheldon's eternal child coming out to validate his stance through maintenance of inner harmony.
Penny: Well why donít you go to a coffee shop?
Sheldon: I donít drink coffee.
Penny: They have other things.
Sheldon: What do they have?
Penny: I donít know, you know, cookies, pastriesÖ
Sheldon: Pastries such as bearclaws?
Penny: Yeah, sure.
Sheldon: I donít like bearclaws.
Yet again, this is outwardly judgmental behavior: when paired with inuition, xNxJ types are among the more naturally capable types at rearranging the thought process of a conversation to work in their favor. Sheldon forsees the line of questioning that is approaching, steers it in his favor, and then closes the door with an outwardly conclusive statement (that is another validation from the tertiary Fi to boot).
I like the distinction of Sheldon as an INTJ.
As a side note, Penny here is almost exclusively asking questions to get down to specifics: a common reflection of Se. When she does make statements, they are very observational (ie. non judgmental) in nature (eg. "They have other things"). Again, this is evidence of the xxxP distinction.