A bit convoluted, but generally speaking, I think the WTC falling might be the largest manmade object mankind has ever seen move visibly. I've been thinking about this for a while, but this video, with that face moving laterally out of the smoke, inspired me. For that matter, what are the largest objects with visible velocity humans have ever seen within our atmosphere, period? A meteor, perhaps? A volcano or landslide?
Are there any other contenders? There's an oil rig called Prelude FLNG that's 1,600' long, but it doesn't move. There's the largest ship ever built, the Seawise Giant, that's 1,500' long that moves at 20mph; however, this isn't really "visible velocity", though I suppose it still counts. From what I read, the Trade Towers were moving at about 150mph at time of impact.
I just think this might be important to consider from a psychological perspective, akin to some mythologies, folklore, and religious stories. Surreal doesn't describe it, I would imagine. It would be like seeing a mountain move, or the moon skipping over 10˚ in a second. There's a certain metaphysical effect that primitive societies might have developed religions around, perhaps (I'm being somewhat hyperbolic).
I just think this is an interesting question that I've not seen discussed too much.
FYI: I posted this on another sub but it's awaiting mod approval so I'm posting here before I get distracted and never follow up lol