Best type of bike lock (materials, style, etc.) I recently lost the keys to my lock and had to borrow a bolt cutter to free my bike. It was a bit of an eye opener to realise a) just how easy it was for the bolt cutters to cut through my D Lock (<10 seconds), and b) that nobody on a busy town shopping street batted an eyelid at me doing so. When choosing a new lock, what should I look for in terms of build material or design that would make it harder to cut through (using relatively small/discrete bolt cutters)?

Get the kryptonite fahgettaboutit - as small as possible. Kryptonite 997986 18mm New York Fahgettaboudit U-Lock,Black Mini

Best lock out there. Because the shackle is 18mm thick, I don't think you could cut it with bolt cutters, so you would need a hydraulic jack -- but the opening is too small for a jack to get through -- that's the main reason to go for a smaller lock, so there is no room for the jack; or, a disc grinder to get thru it.