I'm only going to address what I've completed, and ignore the other two.
Liminaut wrote:3) Shizune
I think this could work if Hisao learns to provide more active support. I could see Hisao becoming the steady, nurturing force in Shizune's life that her parents could never provide.
Depends... Shizune never seemed too involved in the relationship. Once they started dating, nothing really changed. And at the end, they go their separate ways, so it depends on whether or not she and Hisao remain determined to be together, which, given the uninvolved relationship, wouldn't be too likely.
4) Hanako
Hanako has some deep, deep issues; a kiss and a couple of muffins won't fix that. The good times are going to be great but the bad times are going to be awful, and the more Hisao loves Hanako the worse the bad times will be. There's only so much heartache a person can take, and I think Hisao will have to bail sooner or later, for his own sake.
Well, at the end of the story, Hisao does state that they still have a lot to work through, but they both seem to be in a position where they feel there's enough confidence and love for each other that it can happen. If they work through those issues, I'd say they have the best chance of anyone (as far I as know) in having a long term relationship. She's the only person she's ever been in a relationship with, and doesn't know anyone else, and might not get to know anyone else. But as long as she truly loves Hisao and Hisao truly loves her, which was established, then I'd say that there's a better chance there than anywhere else.
5) Rin
Hisao couldn't handle Rin's weirdness in the VN. Long-term, not a chance. I could see the two perhaps maintaing a long friendship, perhaps with occasionally benefits, but not a genuine romantic relationship.
I agree a little, although at the end, Hisao concedes that he can get used to Rin's idiosyncratic behavior. At the end, it seems both Rin and Hisao have come to terms with each other and recognize what works and what doesn't, so if they keep up this understanding of misunderstanding, I'd say a long-term relationship could work.