Most people will recommend a Kryptonite evolution or New York if you don't mind the weight.

Kryptonite Evolution
Kryptonite New York


I went with the more affordable OnGuard option.
This is what I have used with no problems at all.
OnGuard Pitbull Mini

But if you like a larger size you can use the bigger version:
OnGuard Pitbull

What ever you get I suggest you get either a cable (as is included with the OnGuard locks I linked) or even two U-Locks so that you can secure your front wheel as well.