Okay, well, I'm not gonna tell you which one to fap to, but I'll give a list of their traits so you can weigh the pros and cons.
Usagi (Moon): Blonde, odango, twin-tails, airhead, generally cheery and laughs a lot, clumsy.
Ami (Mercury): Short blue hair, glasses (rarely), shy, studious, is likely to blush if you compliment her.
Rei (Mars): Black-purple hair, traditional and somewhat conservative, touhou, hot-headed and quick to anger, but can be very nice.
Makoto (Jupiter): Brown hair, headstrong and tomboyish, good at cooking, athletic, good at fighting.
Minako (Venus): Same as Usagi basically, but more of a slut.