A derogatory word for a splendid person

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.

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.)

(update)
Saccharine: cloyingly agreeable or ingratiating; exaggeratedly sweet or sentimental. Sickly sweet.

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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