This u-lock through the backwheel inside the frame (Sheldon Brown method).
That lock can be cut with scissors or a pair of wire cutters pretty easily. I recommend a quality u-lock.
I purchased a HipLok and I am liking that U-Lock so far
Than pair that with a cable to connect your front wheel to the lock. A cable like this would work
A key with the ulock is don't buy a long/really wide one if you don't need it. I prefer the Mini U Locks as once my wheel/frame is in there and I am locked to an object there is almost zero room to put a jack in there.
Also look up proper locking techniques, such as the Sheldon Brown method or the Modified Sheldon Brown method.
ALL locks can be defeated. Locks are nothing more than a deterrent. Just be smart when locking up. I usually do my best to lock in a spot where I can keep an eye on my bike, or I just park inside whenever I get a chance. When parking inside, still use a lock if possible, as i know some people that have had randoms walk in off the street into their office, grab a bike and leave.