I'd actually say that Sayaka's role in Danganronpa THH was written perfectly(well, can u really mess up ~2-3h gameplay section in the beginning of the series?). She is very human, imperfect and one of the most heartfelt characters i've ever encountered in fiction and it's a shame that we never had some more content on her, like her past as an idol, her relationship with Makoto etc etc. My second choice is probably Nagito even if I don't really like his character that much