Out of all, Mars seemed to capture my interest the most. Her design in my opinion looks the best as well as more fitting for a galactic style group. Also, I think she is overall a great commander for Team Galactic and based on the anime has this relaxed yet rough-edged personality, which seems to outstand for me the most. Plus, her Pokémon Purugly is one of the best she has, disregarding its "ugly" look, acts out to be very tough with attitude and shows strength even though it is looked down upon by others.
Thinking back, when I faced Mars the first time in Pearl, her Purugly actually gave me a competitive challenge. I believe this is what made me favor Mars above Jupiter, Saturn, and the rest because it was my first "challenging" match against any of the Galactic commanders and it's just a memory withheld.