I would generally say look for an old Raleigh MTB, since they were the most common and there's still thousands of the things around now. Dawes also made some nice steel MTBs as did Claud Butler, both much less common than Raleigh though.
1994 Raleigh Sabre.
These welded frames are 18-23 Hi-tensile, nothing fancy but they are strong, and have comfortable geometry. One plus point is they have long chainstays which is good if you want to fit a rack.
1991 Raleigh Highlander
The more expensive Raleighs were lugged 501 like this, or very rarely even 531.
The chainstays on these are slightly shorter, but the top tube is longer than the welded frames. If you have a short upper body the welded type frame will probably be a better fit, as the lugged one has more reach. The Reynolds tubing bikes are a bit lighter, but the difference isn't massive.