About a year ago I bought an Atari 800XL. I didn't have an A8 back then but it got me interested ever since I learned about them in the 2000s.
I adapted a PSU for it and connected the SVideo signal, but the keyboard is unusable and needs to be replaced. It's a common problem with them. I haven't wanted to spend the money, so I haven't touched it since.
In the x86 realm my oldest are a couple of 486 boards and a 1993 Pentium board.
The ISA-only 486 has battery damage. I desoldered a bunch of parts but still have more to go. The parts I removed are stuck into a sheet of paper labeling where they go, but I got sidetracked. I hope I don't lose/confuse that mess before I finally get it back together. Some of those parts might also need to be replaced due to how contaminated they are from the battery.
The Pentium is 2nd oldest, I think it's the original Intel socket-4 board. It improves on the 486 by using a Dallas chip. I bought a replacement for that chip and I think the board works but haven't tested it thoroughly.
The VLB 486 board is newer than the Pentium and uses a coin cell battery. It was an eBay bargain a few months ago, it was having some flaky issues but after a lot of chip reseating, deOxiting, and some resoldering, I *think* it's all fixed and reliable now.
Everything else is socket 7+ so I'd class them as "modern" from a long range point of view. Things got more standardized pretty fast at that point.