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 is a short-tempered jerk, but he rarely does anything more than react to Otto and the gang's shenanigans.
If an adult is telling you a certain area is too dangerous, listen to them! Otto finds out the hard way when he breaks his leg.
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 LeParkour is now, it's hilarious how ahead of the curve Sam was.
Also from Sam, he used the term sick to describe something similarly to how it's used now...only to be met with disapproving reactions from the others. If it had been a few years later, they would've had a different reaction.
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 to trying to explore a restricted forest during in a blizzard and nearly freezing to death. Their closest encounter with death is when they went to get Twister's camera during the eye of a hurricane and it's 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.
MacKenzie Benders, she was an overly violet and needlessly antagonistic Bratty Half-Pint with a super annoying voice who everyone feared (even her own parents!) meaning she always avoided comeuppance no matter what she did. Granted, she was probably written to be a scrappy on purpose.
As mentioned above, Reggie's occasionally stuck up persona and obnoxiously petty levels of Straw Feminism can make her an irritating Designated Hero.
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).
Lars in the episode False Alarmed. He basically ends up the subject of a torture porn episode where he is constantly punished and yelled at for something he didn't do. Naturally we're supposed to sympathise with him... except when Twister and Otto come forward because they feel bad for what Lars has been going through, Lars ends the episode not by showing gratitude but by... mocking them for doing the right thing and getting him out of trouble. After the episode where he and his gang help them out in the sand castle and where he even shows that he does love Twister (albeit when he was the victim of a pretty sick prank...) this just makes him seem like an irredeemable Jerkass.
And speaking of said mean-spirited prank, the characters themselves. They decide to prank Lars by faking Twister's death and film Lars's My God, What Have I Done? speech... then when they reveal it was a prank, watch the footage over and over while making Lars and his gang buy them burritos. Dude, Not Funny!.
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.
Otto riding on the "adults-only" course after a guard told him it was forbidden, and that if he caught the kids trespassing he would put up embarrassing photos of them. He ends up with a broken leg and his friends are saddled with chores for coming along, racing him, and risking their limbs.
Likewise, rather than rest up, he keeps trying to do sports with a broken leg. Even his friends think that he's going too far.