Illaoi - her tentacle are pretty easy to work around when you're used to her, but for inexperienced players with bad positioning? They're hell. Even ONE vessel can easily get you a kill on that 6/0 opponent no matter who he is;
Warwick - he might seem stupid here, but WW is probably the ultimate noob stomper: trash against experienced players, immortal against new people, especially when they want to act cool and pick hard champs since they don't respect his sustain and damage;
Heimerdonger - c'mon he's the donger, should I add more? Ok, no new players knows how to play against his unholy trinity of turrets and even when you know how you still need to be extra carefull to make it work;
Cho'Gath - no one respects his damn ult... NO ONE... also no one plays around his silence, making him broken against newbies;
Trundle - ok this guy is not on the weak side like the others, but he stomps so hard that most new people just don't know how to deal with him with his sustain, his ad steroid AND reduction on q, his w and e which pretty much mean: as long as you need aa I can fuck you and an ult which makes him immortal;
Renekton - he might seem easy to play right? He's harder than he seems at first... but he also hits far harder than what anyone would expect when played the right way... also most newbies have only fought BOT renekton that sucks hard, so they're in for a big surprise;
Darius, Garen, Morde - I place them all here because they have pretty much the same reason to be here: new playes just don't understand how to play against them, they get hit by the full garen spin withouth fighting back, they let morde murder them with his 3rd q and fight darius i his outer q range;
Talon - he's probably both the easiest and hardest assassin imo: he's pretty easy, but when you fuck up you fucked up, no buts about that, ok there is one: inexperienced players don't know how to punish a mistake, so with talon's high burst they'll get scared and let you murder them;
Katarina - she's pretty strong and might seem quite easy, she's not: she needs a good understanding of the game and of positioning and a great timing too. That said since inexperienced players don't know how to stop her she can easly stomp them;
Yasuo - He's just a pain to deal with no matter how good you are, he jumps around too much and it's hard to gauge the exact damage he can output at a given time, also new players don't know how to play around his windwall, which makes him impossible to kill;
Brand - Didn't expect the most awesome fire spirit of the league here? Well he's a beast against new players: high burst, high sustained and incredibly strong aoe. Even if he lacks mobility, inexperienced players don't know how to harrass or how to poke, so he gets his safety from that;
Blitzcrank: Ever seen someone play around the guy's hooks? If the answer is yes, then you're playing in hig plat +, if the answer is no chances are you're stuck in gold or below. Inexperienced players just don't now how to play around him;
Nasus - He's a god under every aspect but inexperienced players don't respect his sustain and don't respect his damage later on, also they tend to fight in his spirit fire (it's called like that right?) and fight him 1 on 1;
Tryndamere - inexperienced players don't understand how to stop split-pushers in the first place, but trynda takes it up a notch by becoming immortal so they just send 1 or 2 people at a time and just die horribly;
Riven - she's hard to play and all of that, but she also hits far harder than people expect her to, also inexperienced players don't know how to play around her all-ins;
Soraka, Nami, Sona - the sustain they bring to the lane combined with theyr poke is annoying, most inexperienced players don't know how to fight when they are in a team and just let them live and heal everyone for free;
Kha'Zix - too many idiots face the bug in a 1 vs 1 scenario when there's nothing around, just to get slaughtered... inexperienced players are in for a nightmare;
Twitch - his DoT might seem weak early game, but most inexperienced players don't know hot to engage/disengage, so they will easily get the full stack before disengaging or escaping, meaning they will eventually jsut get destroyed whn the rat reaches leve 6 and unlocks his full assassination;
Udyr, Rammus, Hecarim, Volibear - inexperienced players just let them run up to them and murder them, they also don't place wards and try to fight them 1 on 1;
Not everyone of them brings some kind of unique feature to play around like Illaoi, heimerdinger or kha, but most of them require specific gameplay to be countered (like renekton who needs to be outscaled, or nasus who needs to be shut down HARD in lane, or brand who needs to be punished every time he's out of position which with his medium range means he gets out of position pretty often).
Like you might have noticed most of these champs have some kind of assassin's tool (I mean half of them are assassins) and/or a range where they're gods (Illaoi, heimer, trundle, riven) because assassin's in general are the hardest to counter for inexperience people expecially, and having a particularly strong range means that the enemy must know how to play around that range and that knowledge comes only from experience (which inexperienced players lack). There are others like them, but they're pretty much the same iterations between the ones I mentioned.