My opinion on who should be with Kazuya is based off of "critical factors for happiness", basically what is essential for a character to achieve fulfillment/happiness in life, and the idea that I would like to maximize every character's happiness. Listed below is what I consider each persons critical happiness factors based on what I've observed throughout the series.
a. Kazuya
1. Have a romantic relationship with a girlfriend/fiancé/wife
2. Have this special person reciprocate his feeling, and mirror the level of passion/affection
3.Have this person accept him for who he is
4. Have a large family
5. Dedicated and emotionally available/intelligent
6. Support Chizuru in her future endeavors
b. Mami
1. Have the ability to lead Kazuya on, just in case she ever decides to fall in love with him, or if she needs a shoulder to lean on
2. Get revenge on Chizuru, Ruka, and Kazuya for standing up to her
3. Drive away the other woman
c. Chizuru
1. Become a great actress on par, or exceeding that of her Grandma
2. Improve herself
3. Maintain a relationship with Kazuya
d. Ruka
1. Acquire Kazuya's love
2. Become a girlfriend/fiancé/wife/mother to Kazuya
3. Support Kazuya in his endeavors
e. Sumi
1. Improve herself
2. Overcome her social anxiety
3. Help others
4. Maintain a relationship with Kazuya
I then pair this with how many "critical factors for happiness" could Kazuya x [insert name] fulfill in a list below.
a. Kazuya x Mami
Mami has the capacity to fulfill the requirements for being a significant other, as well she has expressed a desire to have a large family, but you're a fool if you think Mami will be faithful to Kazuya, or accept him for who he is.
Mami fulfills Kazuya's 1 & 4 requirements
b. Kazuya x Chizuru
Chizuru fulfills a lot of Kazuya's requirements, but due to fairly incompatible personalities and Chizuru's line of work, not all requirements will be met. Chizuru will need to travel frequently to be a famous actress, meaning Kazuya will not be able to take over the family business, nor will he be able to have a large family, at least not unless Chizuru gives up on her actress dreams. Even then Chizuru does not display affection the way Kazuya needs, this will cause him to cling to her even more, and at some point, her independent personality will need to push him away, leaving him to feel rejected. Chizuru would allow Ruka to hang around, and would encourage Kazuya to maintain his friendship with Sumi. Chizuru would also try to extend an olive branch to Mami, but I don't see that lasting.
Chizuru fulfills Kazuya's 1, 3, & 6 requirements (may fulfill requirement 2 as well)
c. Kazuya x Ruka
Ruka fulfills all of Kazuya's requirements for happiness. The more she can be there for Kazuya the happier she is. She is affectionate, emotionally intelligent (something rare for this series), emotionally available, and would even allow Kazuya to support Chizuru in her future endeavors, and would lend a hand to help Chizuru. Ruka would allow Kazuya to keep up with Sumi, but would not allow Mami anywhere near Kazuya. In addition, Ruka has displayed an strong sense of perception, intuition, and is a shrewd negotiator, making her of value to the Kinoshita family business. Ruka has also expressed great interest in starting a family.
Ruka fulfills Kazuya's 1, 2, 3, 4, 5, & 6 requirements.
d. Kazuya x Sumi
Sumi almost fulfills all of Kazuya's requirements, and those that she does fulfill, she excels in. Sumi is super smart, and would make an invaluable asset to Kazuya's family business. Sumi is a hard worker, and with Kazuya by her side, would overcome her only weakness, her crippling social anxiety. Her only drawback is that she seems too reserved, conservative perhaps, and may not desire any children, although she would likely compromise with Kazuya and have a single child.
Sumi fulfills Kazuya's 1, 2, 3, 5, & 6 requirements
Operating under the assumption that Kazuya falls in love with whoever he is paired with, along with the goal of maximizing overall happiness in all characters, Ruka becomes the best choice.