chain locks, with welded hardened links with captive pins. about 1.2 m is good, giving you options to wrap rthe frame and two wheels around a decent immovable object. I like to lock up to large gas pipes or electric high voltage cables for extra security against having the chain support cut...

a 1.2 m chain weighs about 7 kg, same as the bike.

cable locks cut too fast, U locks just take a dip in a jar of liquid nitrogen and a hammer blow.