I ended up voting for Tomo... Not sure why I haven't even watched the series, but here was sort of my thought process of why I like certain female characters over others.
It was between Tomo and Nami. My first thought was that Nami would be more able to beat me up, but then I was like... Saeko and Nico could probably beat me up if they wanted to... I guess it comes down to being more mature? Nami, although pretty level-headed and rational, can sometimes get swept up in emotions. But then I was like... is Tomo more mature than Nami?... ehhhh.
So I figure my decision ended up being about who would be less violent. Saeko and Nico probably wouldn't resort to violence if they didn't need to. Whereas Nami has shown to be violent to keep other people in line, like punching Luffy/Sanji/Zoro/Ussop in the head for acting silly.
And I guess that says something about why I either prefer more innocent characters or more mature characters, because there is more predictability about how they will act. Here I'm assuming that Tomo is an innocent character, but I could be wrong, since I haven't seen the anime. xD
P.S. Keep up the polls, this is turning out to be more interesting than I thought it would be (for me at least). =D