I agree with the above reasons for choosing Rena. Rena is a serious choice. I don't view her as a joke entry in anyway shape or form. (Rika maybe not so serious). Rena has the benefit of not knowing she's going to live another life and does what she does on her own accord (or due to the curse when she goes psycho). People talk about GAR, but GAR means different things to different people. Conviction, belief, badass, style, and guts make one GAR. Rena has these things
Conviction: She stands for friends when she is sane. She stands for her alien parasite theory when she's crazy (and oddly still a rational crazy woman).
Belief: She believes in her friends, even when one is about to smash her face in with a baseball bat. She has all the convictions of Keiichi when they are sane.
Badass: She swings a mean cleaver. Sharp side, handle, or blunt side she still makes an impression with that thing.
Style: Related to her badass and guts, when she used them, she does so with style. She probably has the best fighting style of all those in Higurashi (save maybe Akasaka).
Guts: It takes a lot of guts to laugh in death's face. Even more so if you are a normal human. Now laughing in death's face while telling death off, and being able to give a smug look just before you get killed? Now that takes guts.
Also the afore mentioned not having to go gar for her is a plus. Though I'd happy go gay for her if she'd take me home for it.