Huh, never really noticed that Rein wasn't up there but I don't look at the tier list often. I do disagree with Black Knight being in tier 1 however. His speed is being overplayed a bit and his magic fragility sets him up as just another Ike wit extra HP and more speed, without being as mobile really. His stat spread is really similar to Ike but instead of having a set made to use Aether and set up kills and self restoration, he sets himself up for just...overkilling on one attack.
When a single unit often needs to fight twice, or be used over the course of chain challenges, Ike with Aether and Hector with Sol are inherently very useful to avoid running an actual healer.
Essentially the Black knight is an armored Ike as far as the overall stat setup is and legendary weapon, just with less attack for killing targets on the counter and he's less likely to run QR like Ike to ensure he kills ranged bait targets.
Is he good? Yes. Is he as great as people think/better than a DC Zephiel? I do not think so.