ALL locks can be broken!!!
The best locks are D locks or U locks they are the same thing, and the smaller the U/D the harder it is to break the lock.
Wire cable locks are very good too and a long cable can be really useful when trying to lock up to a fence or tree. 
Chains are the second best option after the D lock but can still be broken, they are very heavy and chips the paintwork on the bike.
The Abus steeloflex locks are ok but aren't as secure as you might think, they look pretty solid but they can be cut with bolt cutters quite easily. 
Brands like Kryptonite and Abus have security ratings which are pretty meaningless in practice. Check out the Kryptonite locks at http://www.kryptonitelock.com/, they cannot be cut, but will probably cost you the price of 2 Chinese bikes.
The best place to store your bike is indoors at home and never leave it unattended outside - not really a practical solution I know, try to keep it in your eyesight if you have to lock it up.. Try to lock your bike through the frame and wheel if your wheels are quick release then make sure they are both secured. You can buy replacement security bolts for the quick release. Always lock your bike to a static object but finding suitable spots is sometimes hard. I have heard of the fence being cut to get the bike so the thief can take the locks off in their own time later on.
I have taken a look at the locks available here like the Giant D Locks and personally I wouldn't use them the chains are especially weak looking.
You can really lower the chances of your bike getting stolen by locking up properly with the best locks available.
Anyway, happy riding.