In my opinion? I can't just pick one! I'll pick 4.
Darius- A Darius will end up pulling in you using Apprehend, brutally damage you with Decimate, slow you with Crippling Strike, then kill you with his ultimate, Noxian Guillotine(a professional Darius could solo and entire enemy team, and win!).
Fiora- A Fiora will charge at you with lunge, and if you flash out, she does it again(!). Greatly damages your with Burst of Speed, makes your abilities/attacks useless against her with Riposte, and she can turret dive/initiate with her ultimate, Blade Waltz, without getting hit/damaged( a professional Fiora can receive a solo triple/quadra/penta(!), while barely taking any damage).
Katarina- A Katarina can damage and mark you with Bouncing Blades, then use Sinister Steel to consume the mark and deal even MORE damage, and she can instantly jump to your location within the range of Shunpo and reducing damage for a few seconds. And lastly, her ultimate, Death Lotus, has extreme base damage, that gets even more powerful once she has her build(she can end up getting a triple with the single use of her ult)! So basically, Q>E>W>R = you dead.
Diana- A Diana can damage you and mark you with moonlight for 3 seconds using her Crescent Strike, pull you back in when you try to escape using Moonfall, and when you initiate her when she's at low health, she can use Pale Cascade to make a shield and make 3 orbs that damage you on contact(the joke's on you!). And lastly, her ultimate, Lunar Rush, instantly makes her jump behind you, and if your afflicted with moonlight and running away, she does it again(she's basically Blitzcrank, Akali, and Janna all in one!)!