To start with, I suppose I'll throw some love for my favourite character. (I don't really have a least favourite, but I guess I like Yamada the least. I still think he's okay, tho.)
It's Junko! I honestly think she's one of the most entertaining characters in the series. For those who are yet to experience the game, you'll see what I mean.
Extreme DR1 Spoilers:
She doesn't have much screentime, but all of it is just wonderful.
If we're talking runners-up, I'll go with Kirigiri and Monokuma. Turns out Monokuma is really fun to doodle, who'd have known?