Katarina: Character-wise, she's vicious and bloodthirsty with a hard passion for her duty. Yet as ruthless and militaristic as she is, she remains to be sultry and sexy, and not even intentionally (doesn't say or do anything overly sexual, like some characters who actually try, i.e Janna, Ahri, etc). Design-wise, she's gorgeous. Hot bod, dresses for the kill lol. Gameplay-wise, just too fun. She gets in, does her work in a flash, and leaves everyone going "Whoa, what". Her animations and sounds are also really cool.
Whether gameplay, her design/model, or her personality/lore, this character feels all-around tailored just to my tastes. Love her.
(p.s for what it's worth, not a post-remake Katarina fan talking here. Loved her even before that, but it's double as much now <3 )