For me, it's the Mighty 486 DX! Just look at all the incarnations of a 486 based PC there can be out there....
1989 - 486 DX-33, 8MB of RAM, DOS 5.0 w/ Windows 3.0
1992 - 486 DX2-66, 16MB of RAM, DOS 6.0 w/ Windows 3.1
1994 - 486 DX4-100, 32MB of RAM, DOS 6.22 w/ Windows For Workgroups 3.11
2000 - AMD 5x86 486/133, 128MB of RAM, Windows 98 SE
I don't think there was a single Intel based CPU with that long of a shelf life on the modern desktop market, or that could at least be used that long running the latest (or darn close to it) software.
And if you have a Turbo Switch, it'll cover most, if not all of the 286 and XT ground at least well enough to be useful for old programs from my experience.