Sweetspot (hitbox)

The sweetspot of an attack is the area of the hitbox with the most desirable knockback effect - usually, the most powerful part of the attack. For example, the sweet spot for most of Roy's attacks is the area of his sword closest to his body, while Marth's is at the end of his blade (often referred to as a "tipper"). Notable sweet spot moves include:

Captain Falcon's Knee Smash (forward aerial). This move is one of the best moves in the game, being an extremely strong semi-spike when sweetspotted.
King Dedede's side smash has a sweetspot at the tip or head of the hammer, having extremely high knockback and being the strongest forward smash in the game.
Kirby's Hammer has a sweetspot at the head of the hammer. When hit with the other parts it will have very low knockback with scant KO potential (however, this hitbox has since been made much stronger). Due to the sourspot being bigger than the sweetspot, the move is rarely used in competitive play.
Luigi's Super Jump Punch has a sweetspot just before he jumps near the center of his elbow. If this hitbox hits a character they will take high damage and knockback, plus a fiery effect and a distinctive 'ping' noise, known as the "Fire Jump Punch".
All of Marth's attacks except Counter have a sweetspot on the tip of the sword, making spacing crucial in order to play the character effectively.
All of Roy's attacks except Counter have a sweetspot at the center of the sword, the exact opposite of Marth.
Mr. Game & Watch's down smash at the head of the hammers send the opponent vertically instead of semi-spiking them, though this has been altered as of Super Smash Bros. for Nintendo 3DS and Wii U.
Sheik's up smash has a sweetspot on the tip of her hands which is very powerful.
The end of Wolf's Wolf Flash is a very visible sweet spot (albeit rather hard to hit with safely), electrifying the opponent and powerfully semi-spiking them. Notably, this sweetspot has its own sweetspot, as hitting with the center of the hitbox is a very powerful meteor smash.
All of Zelda's aerials except her neutral aerial and up aerial have sweet spots. Her forward and back aerials are the famous Lightning Kicks, functioning like Captain Falcon's Knee Smash, and her down aerial sweet spot is one of the most powerful meteor smashes in the game (the latter move only has a sweetspot as of Brawl).
The end part of Zero Suit Samus's Side Special Plasma Whip has an easily visible, fire imbued sweet spot. It is also one of the easiest to hit with, as the sourspot will often push enemies into the sweetspot.
Sex kicks are more powerful at their early frames of their duration. The only exception is Dr. Mario's sex kick as it deals more knockback in later frames.
The Home-Run Bat has a sweetspot near its tip, known as a tipper, but is only useful in the Home-Run Contest, because all parts of the bat one-hit KO except near the handle of the bat.
Mario’s Forward Aerial is a powerful Meteor Smash when sweetspotted, and is really satisfying to land.

Sourspot[]

A sourspot is the exact opposite of a sweet spot. A sour spot is an area with the least powerful part of the attack, such the tip of Roy's blade and the "jumping" portion of Luigi's Super Jump Punch. The sour spot, however, does not always have negative effects. For example:

The sourspot of Mario's, Yoshi's, and Donkey Kong's forward aerials (since Brawl, since Melee for the latter) will not meteor smash, but serve as powerful horizontal finishers and can set up edgeguards effectively.
Luigi's sour spotted Super Jump Punch is good at surprising characters who must charge their recovery moves (like Fox and Luigi himself) and stealing KO's due to the small, practically unnoticeable set knockback.
Captain Falcon's sourspotted Knee Smash and Zelda's sourspotted Lightning Kicks have low horizontal knockback and can combo very effectively, notably into the sweetspots of the respective moves.

As shown in the examples above, the sour spot of an attack can sometimes be more desirable to use than the sweet spot. The sour spot of an attack may also feature minimal knockback, which, provided the attack isn't extremely laggy, can allow a character to combo into different attacks, such as with Captain Falcon's Knee Smash. Many of the scenarios where a sour spot can be useful in battle are highly situational, however.