I don't know how to do that spoiler thing, so if you haven't read the manga yet, don't read my comments. Take it for granted that the manga is awesome and come to your own conclusions.
That being said, I thought the ending was the best choice. I agree (along with tazman0208) that, given given the circumstance with Manaka and Toujo, that Nishino was the right choice for her perseverence. However, for me, even if Toujo gave Junpei some indication that she liked him, I still think Nishino is the best, for personal reasons, of course. But for me, the girl as discribed in Nishino's character is almost ideal. First, as already meantioned, she was loyal and dedicated to their relationship. Second, she was attentive to Junpei's needs and moods (even when he himself was not). Third, she wasn't afraid to talk about potential mixups (think v.9 after Junpei thought she was hugging the pasty guy). If it was Toujo, that would've been the end for them. Fourth, she had the right mix of aggressiveness/initiative and vulnerability (the 3-day "my own personal Junpei" trip). Fifth, she did things for Junpei's benefit without seeking recognition (first valentine's chocolate, breaking up with Junpei in the first place, which was a seriously mature move, can you see Satsuki doing that, despite whatever Junpei felt like?). Sixth, she was both cute (Toujo) and sexy (Satsuki). Seventh, she changed for junpei. Eighth, she had a more mature understanding of love, in that she tried to support Junpei without ever 1) trying to force him to follow her, or 2) trying to force herself to follow him. My final conclusion, Nishino was both the most realistic of the girls (v.18 where Nishino was insecure about her figure as compared to the other girls) and least realistic (if there is such a girl out there, let me know! I'd go for her in an instant!!!) I could refer to so many other moments which, litle by little, solidified my decision for Nishino (I love the confession on the pull-up bar in v.16).So Mizuki, you get 10/10 from me for the near-perfect ending (I agree and feel sorry for Satsuki).