I use Kryptonite 2000, one step below the U-lock, and a Kryptonite cable which protects my front wheel.

But strategy is more powerful than any lock. I bring the bike inside and use the U-lock/cable combo for quick stops at the store. Thieves are looking for something quick and easy, if possible.

1) Parking your bike outside in the same spot everyday is like asking someone, "hit me." Be unpredictable.

2) Park where eyes will help you. Onlooking people are a good deterrent, especially security guards (even if they are asleep.)

3) If you park outside, minimize risk. Use both a full cable lock and a U-lock. The tools used to break each kind of lock are different, so a thief must carry both kinds to steal yours. This is not very likely.

4) Be less stealable than the next guy. The old joke goes this way:

Two guys went camping. At bedtime one laced up his running shoes. His partner asked him what he was doing that for, and he said, "Bears." But you can't outrun a bear, said his partner.

"I don't have to outrun the bear. I only have to outrun you."

5) Park inside.

Oh! I remember a version of the Kryptonite U-lock (2000?) which is much smaller. This is both lighter and harder to break, since it is a small "U" with no room to jack or pry it.