The most difficult characters for mine to play. . . hm.
Renji Watanabe - I'd have to say he's difficult for me to play because he's immature and acts childish and is way tooooo energetic. He is diagnosed with ADHD which makes it much more harder for me to play with him and also researching ADHD on the side to make sure I'm portraying it correctly without offending anyone. He's a silly green haired guy that gets called seaweed. Currently I have him shelved.
My easiest character to play would be..
Tsubaki Watanabe - I've had her for almost 2 years now. She's my most developed and lore driven character she's also a bit similar to me personality wise which makes it easier and faster for me to get into character with her whenever I'm on her.