ABUS Keyed Chain Lock Steel-O-Chain 110cm
If you know anything about us, then you know we believe one lock just isn't enough. No bike is un-stealable, but the more time-consuming your pony looks to unhitch, the more likely you two will ride home together in harmonious bliss after enjoying that soy latte at the coffee shop where the cute barista works who got you into bike riding in the first place, and is always asking how your cat's doing.
So get two locks. Get three locks. We just recommend one of those locks be a chain, and nobody makes better links than Abus.