This lock's 18mm alloy-steel shackle is as strong as the bars in the prison for which it's named. It offered plenty of room to secure our wheel and frame to a railing, and its hexagonal shape prevented our bolt cutters from getting a firm grip. The lock submitted to our hacksaw in 3:32 and our grinder in 1:18, but still proved to be a tough, portable option. A close second. [$90; blackburndesign.com]