If the security of your bike is of the utmost importance for you then the XLC U-Lock Sundance and Cassidy III LO-S05 is the obvious choice for thief proof protection. The LO-S05 features an X2P double bolt as well as a Z cylinder including BumbBlok.
LO-S5
Double Bolt
Hardened steel double rubber coating
Rotatable dust protector
Lock cyclinder protected against drilling and picking