Of course everyone here is going to think she's great because we love Prue,
, but the truth is everyone who's not that big a fan of the character like we are only sees the oldest, most responsible sister who was more of a mother to the girls than a woman herself; responsiblity has never been a major selling point in popularity for characters...usually audiences want someone who's fun and out going or who's got a big romance going and this and that and honestly - Prue had none of it. Andy died in season 1, she was always uptight until around season 3 and then she died at the end of it...there just wasn't a lot of time for her to make up the ground she'd sort of lost. She often came off more as a mom figure and mother figures are never the most popular cast members on a show.
I really think her time on the show hurt her in respect to everyone else, Piper and Phoebe had 8 full years to win everyone over with their epic loves and their spell casting with personal gain and such...Paige had a total of 5 years and already came in as this fresh new character who used magic for personal gain, who had half the powers of one side and half the powers of another so she was very unique in her own right and then she was very much a free spirit who didn't do what she was told always - things like that appeal to younger audiences who were the targets for the show. Poor Prue only had 3 years and for the first year and a half she was very uptight and always the one saying 'We shouldn't do this' or 'No magic for personal gain', again, she was the mom of the show and they're very often the most under appreciated characters in a family setting.
I've always loved Prue personally, but unfortunately as far as mass audiences go she just didn't have enough time and enough great moments on the show that were fun and carefree for people to remember her by and for those fans who only knew the second Power of Three they hardly knew anything of her at all thanks to them never mentioning her at all, sure occassionally Piper would toss something out there but they kept her pretty bottled up on the Prue talk...I blame Brad Kern in part, but that could just be some of my agression towards him. Not sure if this helps you much Farvarine, but it's just some possiblities as to why people might think of Prue as not that popular of a character - lack of years to win fans over, not talked about after she passed, always the mom of the group, and not enough carefree moments that would really stick out to those who joinned in the later years that actually bothered to look up and find out more about her.