It's probably Yoshi, Ganon, Link, Bowser or Diddy, though no character is easy to master. ICs? What's hard about memorizing a ****ing button input? I could CG every high tier with my eyes closed after 1 and a half hours of practice back when I used to main ICs, and I could CG the top/high tiers in actual brawls after about a day. Desynching, yeah, sure, but I found it to be mostly useless as the player using the simpler character(IE, a weegee, DK or MK) doesn't have half as much to think about.
Bowser is a character that comes to mind because you have to know the character up and down, take him apart and put him back together before you're even decent with him, you have to have precision button input just not to get punished and you have to learn possibly the hardest CG in the game.