For us, it's different every time. In order to keep our schedule from skewing too far toward nonstop M&Gs, we allow our kids to each pick just 3 characters that they each want to meet, and I work it into our touring plan and get any necessary FP+.
For our first trip, my daughter wanted to meet Minnie Mouse, Donald Duck and Aurora (who has always been her favorite princess, even though she's seen Sleeping Beauty precisely one time), and my son wanted to meet Mickey, Goofy and Stitch. We were able to knock those out (plus meet some additional characters -- some at group M&Gs and some of which we stumbled upon as we toured) with visits to the MK's Town Hall (back when it had the princesses), Epcot's Character Spot (back when it had the Fab 5) and the 'Ohana character breakfast at the Polynesian.
Last time around, my daughter wanted to meet Daisy Duck, Belle and Anna/Elsa (frankly, she was at least as excited about meeting Daisy as she was about Anna and Elsa!), and my son wanted to meet Mickey (again), Goofy (again), and Peter Pan, so we did Pete's Silly Sideshow, Enchanted Tales with Belle (w/FP+), Princess Fairytale Hall (Anna and Elsa side, w/FP+) and the Peter Pan M&G (arriving right when it opened).
I think that in general, girls tend to be more interested in meeting the female characters, and boys skew toward the male characters. For that reason, venues that offer two different options or a mix of male and female characters (like Pete's Silly Sideshow or the M&G areas in HS) are particularly handy for our family, because we can split up if necessary to get to the characters in which each child is most interested.