Old chrome moly rigid MTB. Trek 700 or 800 series, Specialized Hardrock, Giant Boulder or Yukon, Schwinn Frontier, etc. Cheap, tough, comfortable, easy to find, all use common parts so no ordering oddball components over the internet. Many came with 36-spoke wheelsets adequate for commuting. Not the lightest framesets out there but you are looking for a workout on a short ride anyway.