I'm not sure it's necessarily about romantic love; they never use the word "love" in the interview. Amir was asked "Of all of the other characters in the show, who do you think he cares about the most? Who will he go above and beyond for versus anybody else?" Answer: "I'm not gonna answer that."
I don't necessarily think that it being "too big of a reveal" means that it is definitely romantic, but knowing who it is could involve a major spoiler for the plot in S2 or S3. It could be romantic! But it could also be familial or platonic, which is why I think it's most likely Vox. The other obvious answer would be his mom, but fans who have read a Hazbin wiki would not see this as a spoiler (although Amir might not be aware that Alastor's love for his mom is already common knowledge).
We know that the plot is partially driven by Vox and Alastor's "complicated and sad" backstory and relationship, and that Alastor's backstory will be revealed in S2. So if Amir revealed that Vox is the one person Alastor cares about the most, it would spoil more about the impact of their relationship with each other, especially when you put in context of other information we know abut Alastor's backstory and S2.
We've seen from the "Day in the Afterlife" that Alastor struggles to maintain friendly interactions with the citizens of Hell without them running away from him in fear, and he looks a bothered and a bit dismayed by this. And this comic juxtaposed these terse interactions with his antagonism with Vox; perhaps their falling out is especially painful for Alastor because Vox was one of his few friends for a long time. And their rivalry (and instinct to immediately antagonize each other) could be driven by their mutual need to remain in each other's lives one way or another.
I can't seem to find the source at the moment, but I recall Vivzie recently saying that what Alastor needs is different from what he thinks he needs. What he needs might be to trust others and develop healthy relationships with other people, and his falling out with Vox could be one of the reasons why he struggles to trust others.
"Care" might not even mean that Alastor platonically or romantically loves Vox still; it could mean that Vox's role in Alastor's life is as essential to him as breathing, and he would do anything to keep Vox's attention (and keep him alive so he can continue giving him that attention). From Ep 3, we see that Alastor wants to live in everyone's head rent-free all the time, as evidenced by him asking Zestial to share the theories about his disappearance and his reaction to Carmilla's dismissal about whether everyone "wondered" about his disappearance.
Meanwhile, Vox clearly cares and thinks about Alastor all the time and will drop everything to focus on him (and vice-versa). I can see Alastor saving Vox's life just to not lose someone who devotes that much energy on him, even if that energy is used to plot against him and stalk him relentlessly (which Alastor encourages, because he doesn't know how to have a healthy relationship).
That's one of the reasons why I like both Radiostatic and Radiosilence so much; it's an opportunity to explore different types of deep connections between people.