A lot depends on where you are, theft, and the value of the bike.

At the moment, I'm using a Kryptonite NY Lock. It is about a $100 lock (although I snagged it second hand for a bit less than that). And it is beastly HEAVY. My bike also catches people's eye.

For a more moderate bike, in a low risk community, then just about any U-Lock will be a big deterrent.

I can usually snag my frame, wheel, and something solid with the U-Lock, without a separate cable.

Oh, and the bike is rarely left unattended late at night.