Futaro is a very blunt person. Because of this, I don't think he has long-term compatibility with each of the five sisters. I think some of the potential pairings would have issues a few years into the relationship.
From least to most compatible:
Miku: I know, she's really popular, but I honestly think she and Futaro would grow to resent each other after dating for a couple years. Miku does not have thick skin; Futaro's tendency to always say what's on his mind is going to hurt her feelings fairly often. And, because she's so shy, instead of communicating her issues directly, she'll try to be sneaky about it, which will go over Futaro's head and leave both of them frustrated.
Nino: She's too proud. Futaro isn't Prince Charming. He won't treat her like the princess she wants to be. Once again, his bluntness is going to offend her on a semi-regular basis. Hell, that was already happening during the course of the manga. Eventually, she'd realize that Futaro is not the man she fantasizes him to be.
Ichika: While she won't feel insulted by Futaro's general tactlessness, I anticipate that, much like Miku, she'd try schemes and indirect ways of telling him what's on her mind instead of directly addressing him. I imagine that, without semi-regular couple's counseling, Futaro would break things off with her.
Itsuki: This could work. She and Futaro understand each other fairly well. Sure, they'd annoy each other, but not enough to cause any actual damage to the relationship. The only minor issue is their lifestyle difference; Itsuki likes to be quite a bit more extravagant than Futaro. They'd have trouble agreeing on a household budget.
Yotsuba: I honestly believe Futaro has the best compatibility with Yotsuba. She's not the sort of person to be genuinely bothered by Futaro's blunt honesty. She likes simple things and cares more about company than the activities they do together. Also, these two are really good at supporting each other, and their personalities contrast in ways that complement each other. Futaro could use the energy to not isolate himself socially, and Yotsuba could use some stability (the archetypal "yellow and purple" couple.)
