I've always liked Nacchi, and ever since I got into H!P (way back in '99), recognizing her talent and natural beauty instantly.
One of the things I really love about her is her genuine smile, and just overall genuine personality. A lot of the H!P girls 'act' their emotions on camera more, but Nacchi, among some of the other girls, is really displaying how she feels most of the time. Of course, I'm not saying she doesn't 'ham it up for the camera', all the girls do at different times; she just seems more constant when it comes to acting genuinely happy, for example.
I find it really sweet how she's the "big sister" to a lot of the girls in H!P, and does the part really well, you can tell she really cares for her friends and that she knows that as a sempai, she has to be responsible. Don't take the plagarism scandal into account, she's human, she makes mistakes just like all of us.
Besides all that, she's just extremely talented, an all-rounder. She can sing, she can dance, she can act, she's a sweetheart without the 'diva personality', what more can you ask for?