This should be a simple question considering I only started buying vint boards a couple of months ago or something, but I actually don't know if I can remember for sure. Which is probably a bad sign... 😅
I think the first one I bought was a Dell AT102W, but I don't think it was the first I actually received: the courier lost it for an absurd amount of time, to the point that the seller started a claim. They then found it, after he'd already refunded me, so they then delivered it and I had to either send payment again or be a dick (I chose the former because I don't want to screw anybody over).
I'm not certain, but I think the first one I actually received was an Apple Extended Keyboard II, but that also didn't go well: it arrived with a huge bend through the entire assembly, and the seller barely gave me anything back for it, which was really irritating. I did my best to reverse the damage, but it'll never be fully flat now, so I'm probably going to scavenge it for parts to put in another build.
The other one I got very early on (and the only one that wasn't a mess) was a pretty much NOS Acer 6312-K (Data General branded, Spanish ISO layout) that another keyboard enthusiast had previously bought and tried to improve somewhat by adding some foam inside the case to reduce some creaky noises the chassis made from time to time. I liked it so much I bought another two of them, and one for a friend. To be fair, I probably should've stuck with 2 rather than 3, so I might sell one of those, depending on how things go, but I genuinely do love those switches, even though they're clearly not the absolute pinnacle of keyboard engineering. I'm more of a Model F man, as it turns out...