>>107540
Striking and interesting color palletes, aesthetically pleasing outfits which help demonstrate their personality or identity, good hair (from color to length to style, etc.), a nice body in some way (ideally with pleasing shapes),pretty eyes, good symmetry where appropriate (and interesting asymmetrical features to help stand out if you like), clever exaggerations if that's your style, and so on. Overall distinctiveness will help too.
What
>>107542
says will also help, and it will probably help you figure something out.
IMO the best designs often come in the context of something greater like a creative work. Its rare that you see good designs come out of a whim, like some fags random OC or something.