What is the most dependable workstation MB
i think most dependable is often the MB that OC's the best - but run at stock speeds. i know that's not as much fun, but ... that it will take a major OC is an indication of the stability @ stock frequencies.
if you're willing to go socket 775, i think the intel 975 XBX2 is a classic. a little aged, admittedly.
on the socket 1366 side, the Asus P6* variations, e.g. the P6 WS model, stand out. there was an OC review where someone took one of the P6 WS' up to 5 GHz stable.
there's a few custom PC manufacturers that also use the Asus P6 in their workstation.