For me, it's the fact that she's the first character to buck the whole "princess" trend, where the strawhats come across a girl, usually a princess, that helps them out each arc. Pudding was that girl... until she did a total 180 and revealed herself to be evil. And as I have said before, her being evil in and of itself is already a very welcome twist. But the fact that she's a sadistic, trollish dick about it is what makes me really love her character. If she was just simply evil and doing Big Mom's bidding or whatever, it'd be a lot easier for me to be on the whole "Fuck Pudding" club. But this recent chapter just solidifies her as a character I love to hate. Or maybe just love, I dunno lol
Then you have her powers, which to our devious minds, let us picture her inserting herself in pivotal One Piece moments, which alongside her nasty personality makes for solid humor/meme material. I have never laughed so hard concerning a One Piece character in quite some time.
So yea... favorite One Piece character by far right now, hope she holds up.