Abus Granit Plus
This is a really high security lock that's still fairly light. It's big enough to get around street sign poles, but small enough that you couldn't fit something in to lever it open.
It has a security rating of 12 (on a scale of 15), which is seriously good security.
It has a very high quality key and locking mechanism that's smooth and quick to engage, so you won't be fumbling in the dark to lock your bike up.