The ABUS Bordo Granit X-Plus 6500 is a great option. It's not too heavy to carry with you and can be mounted on the bike. I have it on mine and it's great.

This is probably the minimum security lock I would trust to secure my bike. Next step up is a ABUS Granit U-Lock. For maximum security get a nice ABUS chain and an ABUS shielded shackle Granit padlock.