In Swedish there is a word "präktig" that can be used to describe a person that is annoyingly decent, reliable and good in every way. The common translation is "splendid" but that doesn't sound derogatory at all. "Pompous" is the closest I've come, but that is a character flaw and doesn't really fit. So is there an English word that can be used to describe a perfect person that also indicates that he/she is too perfect?
A goody-goody (One who is affectedly sweet, good, or virtuous) is a typical derisive name for someone who really is a decent person, but makes others feel defensive about their own flaws. Ned Flanders on the Simpsons is the most decent human being on the show, but it drives most people (especially Homer) crazy. (He donates a kidney and a lung out of the goodness of his heart to whoever needs them first.) This is also known as a goody two shoes.
A killjoy (one who spoils the pleasure of others) may be so called because he/she disapproves of bad behavior. It may also be applied to a pessimist who just likes to spoil things for others, though.
If a person is truly good, you will not find a derogatory term for them unless it's based on jealousy. A goody-goody is cloyingly good, or seen to be affected, but that is as close as you'll get to a truly good person who annoys others.
Other words are sanctimonious (making a show of being morally superior to other people), pious (devoutly religious), godly (no negative connotation except to atheists), saintly (very holy or virtuous) which may be used sarcastically or in jealously, irreproachable (beyond criticism; faultless), immaculate (spotlessly clean) which would be hyperbolic.
Whiter than white/snow (benevolent or without malicious intent; pure, honest and moral) carries a bit of a defensive tone.
Erroneous labels for a good person which may be misapplied out of jealousy are: canting(affectedly pious or righteous), holier-than-thou (characterized by an attitude of moral superiority; marked by an air of superior piety or morality), pietistic, priggish (a self-righteously moralistic person who behaves as if superior to others), self-righteous, too good to be true.
In English the only (mildly) derogatory words I know of for having no other faults except being too good are Choir Boy and Boy Scout.
In both cases these refer to someone who is either uncompromisingly good in a way that others find constricting or even oppressive, or alternatively, someone who is so good that you cannot act (or speak) normally, you are compelled to act better around them to conform to their expectations which are proper, so you cannot reasonably argue against them.
In English, you usually use some emphasizing qualifier to indicate that you are using the terms derisively, as in "Joe's such a choir boy.", or "Not with a real boy scout like him." Note that the derision implied is very mild and usually carries a hint of admiration as well.
Example: In DC Comics, the always good-guy Superman is sometimes referred to as "the Big Blue Boy Scout", by criminals and even, on at least one occasion, Batman.
And though I have never heard it, I believe that you could also use either "Choir Girl" or "Girl Scout" in the same way to refer to a female and most hearing it should understand the allusion.
If you are looking for genuine splendid sarcasm:
One of the beautiful people: wealthy or famous people whose lifestyle is usually expensive and well-publicized.
If you were to say "He thinks he's one of the beautiful people", you mean he puts himself in a class where (the little) people are expected to fawn over him and that he is faultless. (The faults of beautiful people are quickly overlooked by their adulators.)
(By the way, beautiful people can be taken positively, but it's easy to spot sarcasm with this phrase.)
Saccharine: cloyingly agreeable or ingratiating; exaggeratedly sweet or sentimental. Sickly sweet.
There are also several words with a similar meaning (besides the ones already given) in common use. These all will universally be taken in negative terms:
priss: (back formation of prissy) An excessively proper person; one who is affectedly correct or prim. (where proper means conforming to established standards of behavior or manners; correct or decorous.)
prude: a person who is excessively or priggishly attentive to propriety or decorum; especially : a woman who shows or affects extreme modesty
victorian: having the characteristics usually attributed to the Victorians, especially prudishness and observance of the conventionalities.(mildly negative)
self-righteous: (adjective) confident of one's own righteousness, especially when smugly moralistic and intolerant of the opinions and behavior of others. (To call someone a righteous person, on the other hand, generally is a positive or neutral term suggesting their religious nature.)
If you want something is coarser language, there is this. (Don't look if you are a prude.):
He thinks his shit don't stink: He is so much better than everyone else. (ref)
I believe you are referring to a "goody-two-shoes." The phrase comes from The History of Little Goody Two Shoes, A link to the Wikipedia entry.
Mr/Mrs Perfect is quite a common term. Possibly derived from the Mr Men character of the same name
The Saki story The Storyteller suggests a phrase:
“Once upon a time,” began the bachelor, “there was a little girl called Bertha, who was extra-ordinarily good.”
The children’s momentarily-aroused interest began at once to flicker; all stories seemed dreadfully alike, no matter who told them.
“She did all that she was told, she was always truthful, she kept her clothes clean, ate milk puddings as though they were jam tarts, learned her lessons perfectly, and was polite in her manners.”
“Was she pretty?” asked the bigger of the small girls.
“Not as pretty as any of you,” said the bachelor, “but she was horribly good.”
There was a wave of reaction in favour of the story; the word horrible in connection with goodness was a novelty that commended itself. It seemed to introduce a ring of truth that was absent from the aunt’s tales of infant life.
How does “horribly good” sound to you?
Probably the nearest equivalent to your präktig would be paragon, which means a model or a perfect example (of something). The most commonly-used phrase containing the word would be paragon of virtue, though you could say paragon of uprightness, paragon of probity, etc. Although it's ostensibly a compliment, it's been used in an ironic sense for long enough that to use it at all about someone is to imply that their virtues evoke ridicule rather than admiration.
It depends on whether the person in question is trying to be superior to those around them, or is just a really nice and decent guy. Lots of terms have been suggested for the former (goody two shoes, prude, priss, bluenose, etc.). Ned Flanders almost falls into that category, but he seems to be just trying to live up to his moral ideals (as annoying as that can be at times) rather than deliberately elevating himself above others. Frankly, I can't think of a derogatory term for someone in the latter group.
Would most people rather have Ned or Homer for a neighbor?
You could also use an out of date term that used to mean someone was really special. Words that come to mind would be swell, super or neat. Every time my mom says, "You should really meet this person, they are really neat." I think, "I don't think so."
This approach has the added bonus of keeping some of the population in the dark.
The question suggests an answer; splendiferous
If you look up the synonyms, it seems that the majority are negative.