Kryptonite KryptoLok Series 2 ATB
You can never go wrong with a u-lock. The classic u-bar shape is designed to loop through your frame, one of your wheels, and something cemented into the ground, like a parking meter or bike rack. Of course, thieves can cut through anything, but some things are harder to cut than others, and a u-lock is the best form of bike theft deterrent. Made of hardened steel and locked with a pick-and-drill-resistant 'disc-style cylinder,' this lock resists attack of all forms; bolt-cutter, drill, car-jack, power-saw. At $40, it's at the higher end of budget locks, but it will do the trick.