Look for a used mountain bike, with no suspension. Low key, less likely to be stolen. $75-$175.

Trek 850, 930, 950, 970.
Specialized Hard Rock, Rockhopper, Stumpjumper
Schwinn High Plains
Miyata Path Runner, Ridge Runner, etc.
Raleigh M40
Gary Fisher, GT, and Gaint as well.

Ideally a chromoly steel frame, trigger shifters as opposed to twist grip. Shimano STX, Deore LX, or Exage drivetrain. Shimano Altus, Alivio, Tourney are low end mountain bikes. Altus brakes are prone to have cracked grey plastic spring retainers, which necessitate buying a new brake assembly.