Not particularly special, but for me it has to be my IBM PC Server 704.
Mainly because there's just no way I'm running anything but Amiga OS on my Amigas. (Not even Amiga Unix, yes I do have an original A3000UX)
Sadly it doesn't even work anymore. All three power supplies have given up the ghost and even then I don't think the CPU boards even work anymore.
I got it for free; went to a big computer show in 1995 or 6, and tolda couple of guys they should buy them at $50/ea. They did. The one guy ended up giving me his after running Amithlon on it for a while.