I love Prue because while she was sometimes annoying, she was responsible and put her family above everything else.
I love Piper because she taught me that it is possible to find love - forget Romeo and Juliet, I want to find a love like Piper and Leo.
I love Phoebe because she is strong and independent - even through all her heartbreaks, she still comes back fighting.
I love Paige because she showed me that even an outsider can still fit in once they find the right people.
I know you asked for a favourite, so I'd have to say say Piper, then Phoebe, then Paige and finally Prue.