U-locks are often a hard, un-cuttable metal (an angle grinder'll do it, but you'll shred a hacksaw; you WILL gain attention in either case); however older (pre-2005) Kryptonite U-locks of a few models had a flaw in the mechanism that allowed the 3 second bypass with a Bic pen. Mark Tobias found and published that one, and they brought the locking mechanism from their high end lines down into all lines of U locks, so it should be fixed.

The first thing you need to understand is that all locks can be bypassed. That nobody has an exploit yet doesn't matter; there is one, and if Mark Tobias takes an interest in the lock it'll be dead and gone in 3-4 months. He's rather unique, though; he's fast, and good at what he does. Most of the time, you don't have locksmiths looking at vanilla bike locks with unique, high-end locking mechanisms for specific exploits; and when you do, there's only a handful out of the whole lot that are as good as Tobias, so most will take years to find an exploit. A few of the good ones will find a crack quick.

That being said, I use a Master Lock that I suppose is pick-resistant, but you could eventually pick it I'm sure. I use this with a Kryptonite steel cable, which I want to replace with a custom cut of titanium chain. I think that will be more than sufficient, and I'll tell you why.

The Master Lock, I forget what line it is, but it's one of their fancy new Boron Carbide locks. Try hitting one with a bolt cutter; you might break the bolt cutter. You probably won't break the lock. Boron carbide steel is extremely hard, and it will hold up to a hack saw or a grinder for quite a while. The most aggressive tool you could tear at it with might take several minutes, and will be quite noisy at it.

A titanium steel chain is going to do the same exact thing: refuse access by hacksaw and bolt cutter. Again, you will be here for quite a while trying.

This means I can loop the chain (wrapped in shrink plastic tubing) through my bicycle's rear wheel, frame, and front wheel, and then lock it with an unbreakable lock. Now you can't steal my wheels; maybe you can take the quick release pin, and I will be quite annoyed, and need to spend $15 on a taxi and then $10 for a new quick release mechanism at a bike shop.

But as I said, all locks can be bypassed. It's a matter of time, or of finding a serious lock-specific exploit--again, this rarely happens and it gets fixed fast (unless it's a Medico 3 lock, then the company just covers their ears and wishes you would go away).

As for time, that's an easy one. Put your bike somewhere it's going to get attention. Attention by thieves is fine; bike rack, high traffic area, bolted down under a bright street light, sure. And during the 15-20 minutes they're there with power tools, the cops show up. Nobody wants to squat down for even one solid minute to steal your bike; if they can't grab it and go they're going to steal the next guy's bike instead.

For reference, the absolute worst place to park your bike is chained to a solid post in a dark back alley. When you come back 2 hours later, some guy will have had an easy hour and a half to crack your super high end locks. Sure it takes time, but who's watching anyway? Or, since you're off in the dark alone, they'll just wait in the shadows and then club you and take your bike once you unlock it (that's what I'd do; this is not rocket surgery). Even if you have a combination lock and they spot you, what good is that combination? They can't steal your key? Have you ever heard of rubber hose cryptography?

So yes, basically you want a cable lock that can't be cut without power tools, coated with a vinyl shrink hose, that you can run through both wheels and the frame. Lock it with some kind of unbreakable padlock, like a $10 boron carbide Master Lock or anything titanium-vanadium-steel if you have $10,000 for a padlock. But more importantly, chain it up somewhere that draws attention to thieves so they don't want to squat down for 10 minutes trying to break your lock. I could chain my bike up in a very high crime area and get away with it: the bike racks are directly across the street from a building constantly surrounded by about 30 police watching the street every friday and saturday night (no kidding, they just show up at 8pm and stand guard until 2am). There's 30 cops right there, 50 feet away, watching, for 6 hours straight. If you come down the street with a bolt cutter, you will get questioned.