tatfiend 

IMO probably an old Raleigh roadster would be one of the strongest frames ever made. Relatively thick wall Hi Ten steel tubing brazed together with lugged construction. Also very thick walled steel wheel rims. I have seen photos from Africa of them loaded with hundreds of pounds of cargo or husband, wife and kids.

The Chinese and Indian made copies of the old Raleighs, still available new, should be equally strong but reportedly without the quality of paint and plating used on the old Raleighs.

A low end early 1990s lugged steel frame MTB from a name manufacturer as mentioned by other posters should also be excellent as would a Schwinn Varsity, except for the stock front fork.

If hit by a motor vehicle or run off of the road though I would worry more about myself than the bike. Bike frame survival is highly dependent on the details of the accident and has too many variables to make a definite prediction. Remember that if hit the car will outweigh the bike by at least 50 times, in most cases more like 100x, even for the heaviest bike. Pretty obvious which is more likely to win.