Otto, unsurprisingly. He's highly inconsiderate and vain to everyone around him, and it certainly doesn't help matters that some of his enemies have extremely good reasons for antagonizing him to due to him being Otto.
Reggie, while not to the extent of Otto, occasionally has moments of this too.
Designated Villain: Merv Stimpleton. He's a short-tempered jerk but rarely does anything more than react to Otto and the gang's shenanigans. With this being a show centered around kids, he is constantly portrayed in the wrong for no good reason.
Ear Worm: "Vegemite, Vegemite, we spread it on everything, it always tastes just right, Vegemite, Vegemite, Yum Yum morning, noon, and night!"
The episode where Squid makes friends with an asocial computer geek—don't hang out with them, they have no life!
This is especially bizarre; because Sammy acted just like Oliver when he was first introduced. But to give the show a bit of credit, the crew accepts Oliver and find something he's good at: Keeping score and time.
"Big Thursday": It's okay to beat up your kid's role model as an excuse for being a bad parent.
Twister would often duct tape his camcorder onto his helmet, thinking it's a genius way to shoot his videos while leaving his hands free. About 10 years later, the GoPro is invented which is a small digital camera that can be mounted onto helmets.
In one episode, after having skateboarding banned on the pier, Sam suggests that they substitute running on-foot for skating which everyone immediately shoots down as being lame. Considering how popular Le Parkour is now, it's hilarious how ahead of the curve Sam was.
Informed Wrongness: The shoobies in The Lingos. Their crime is that they think the slang the gang uses is cool and starts saying it. They seem to have freedom of speech confused with copyright infringement.
Jerkass Woobie: Lars in the episode "Falsely Alarmed". It's no secret the guy is a complete douchebag, Pet the Dog moment(s) aside, but really, he got a week of community service for something he didn't do. He was simply in the wrong place at the wrong time.
Twister pretending to have been Driven to Suicide in "Escape From Lars Mountain" can be too home-hitting for those who have tried to take their own lives, know loved ones who have, or been the victim of a similar prank.
Any and every time Otto's rebelliousness puts him and others in danger, which happened multiple times.
Really, anytime one of the kids, usually Otto and Twister, finding themselves in Life Threatening situations and it happened very often. From Snowboarding down a dangerous slope that broke Otto's leg, trying to explore down a restricted forest during and nearly freezing to death in a blizzard. Their closest encounter to death is when they went to get Twister's camera during the eye of a Hurricane and only through luck that parents found them in time before they drowned; seriously, when there's a natural disaster going on, you get to the nearest shelter, and stay the hell there because losing your life is not worth your material possessions, even if they're expensive to replace.
The Scrappy: Ironically, the main character, Otto Rocket, is this to many, for being a rude, lazy, selfish, and conceited Jerk Jock with a massive ego who regularly mistreats his friends (especially Sam). What's more, he suffers from a severe case of Aesop Amnesia, with each episode having him learn his lesson only to see him revert back to being a hyper-competitive jerkass by the next episode.
Strawman Has a Point: One early episode has the city council ban stuff like bike-riding, skateboarding, rollerblading, etc on the pier. While it's pretty obvious that they're Straw-Moral Guardians and Merv is clearly exaggerating his injury, they actually do have a point that it can be dangerous due to how crowded it is. (And in fact, it should be common sense to not skate around crowded areas with loads of foot-traffic.) That said, Ray does point out to Otto and Reggie that they initially refused to apologize for hurting Merv, justifying his complaints (if not all of his subsequent actions).
Values Dissonance: "A Rocket Power Christmas" has Reggie admitting she's willing to accommodate a vegetarian diet for a dog in an offhand comment. Nowadays feeding your dog a vegetarian or vegan diet is flat out abuse, and those who attempt it are considered either morons or monsters. Even at the time the show was made, this was considered a bad idea.