High
Kindness is the requirement, and it functions less like a preference than like a threshold — someone can be impressive in every other way and still be out once you've seen them be careless with a person who couldn't do anything for them. Of the three standards this is the one that ages best: it selects for what still matters in the twentieth year, when the pull has settled and the job has changed twice. Two things to hold. Warmth is easy to perform early and expensive to verify, so a high bar here rewards patience more than instinct. And a high bar can be lonely in a different way: warmth is necessary, but on its own it has never been enough to build a life on.
Moderate
Warmth matters to you without being the gate. You notice unkindness and you don't ignore it, but you weigh it against everything else rather than treating it as an immediate end to the conversation — a difficult person with real qualities gets a hearing. That is the ordinary calibration and a workable one, and it avoids both failure modes: you're not choosing on charm alone, and you're not disqualifying everyone who was short with a waiter once. Where it costs you is in the middle cases, which is where most real people live. Deciding how much coldness is circumstantial and how much is character takes you longer than it takes either of the clearer bars.
Low
Warmth isn't the standard you screen by. Kindness registers as pleasant rather than as the thing that decides it, and you're comfortable with people who are cooler, sharper or more reserved than most — often you prefer them, because reserve tends to come with the qualities you actually notice. This is a legitimate setting and it widens your field considerably: you'll consider people that warmer bars filter out on the first evening. What it doesn't do is protect you. Warmth is the standard that governs how you'll be treated on the day something goes badly, and a bar set low here means you'll find that out by living it rather than by screening for it.