While I reserve the right to be wrong as always, my money for slowest would be VAX 11/725 or MicroVAX I. But a 16MHz 68010 (did they go that high? The 68000 did, but the fastest '010 I've seen is 12 MHz) is likely to be faster, if dhrystone is any indication. The Byte unixbench suite would give a reasonable result.The Sun 2/50 would certainly be in the running for slowest...anything. It's a, what, 16MHz?
EDIT: 2/50 is documented as 10 MHz. Sunhelp Document. (but it's not supported by NetBSD/sun2, apparently....)