Just get the thickest cable lock system you can find and long enough to reach the front and rear tire plus whatever your going to be locking it too; these type of locks are fast and portable because the cable just loops up for storage.

BUT do not use a cable system if your going to leave it for more then 10 minutes because cables can be compromised rather quickly. A U-bolt is fast is you don't care about securing the front wheel, but problem is to lock these up correctly you should remove the front wheel and place it beside the rear wheel then lock the shackle through both wheels and the frame and whatever your locking the bike to, and this takes time.

And if you do live in a high crime area and are worried if the bike is kept outside for an extended time then you actually need two types of locks, one U lock and a chain lock or at the very least a cable lock. Two different type of locking systems means the thief will need two different type of tools that they simply won't carry.

Better yet for high crime areas is just buy a used bike for 25 or 50 dollars and ride it when you know your going to be parking it outside, and use a mediocre lock.