Wow, what a hard choice this is. . .
Although I do find Flaco annoying with his drillshine combos, he's pretty easily chaingrabbed, and his lasers can be powershielded. Marth can be evil with his grab from half the stage away, as well as his impenetrable wall of double short-hopped f-airs. . .but there's (almost) always an opening; it's just a matter of seeing where it is. I don't have enough IC's experience, but I imagine that a properly de-synching, chaingrabbing ICs would probably wreck all of humanity and **** the universe for free. Sheik is cheap, but I've come to accept that, so whatever.
. . .I'd have to give my vote to Peach--it's not just that she's cheap, but she's highly irritating, too. Overwhelming priority and range on much of her aerials and ground moves snuffs out everything you can throw at her. She's impossible to grab, which cuts down comboing possibilities by a huge percent, and she's impossible to kill unless you have a strong vertical KO move. Plus, her d-smash makes crouch-cancelling and close-to-the-opponent-shielding almost useless, which takes away lots of useful tactics that you might have used. And having a spammable projectile with the possibility of grandpas, bomb-ombs, and beams swords just makes things worse.
Wow, Peach is cheap. Frustrating, too.