The one, the best, the u-lock that I most highly recommend is the Abus Granit Futura 64, aka U-Lock 64/120HB150.
It’s small, it’s light, it’s extra hardened. It’s rated by the manufacturer as more secure that some of their other (heavier) u-locks.
Advantages/features of this lock:
- small & light
- locks both sides of the U rather than just one end
- keyhole covered, keeps out crud
- stronger/harder steel than other, heavier locks
- no good bicycle mounting option
- sometimes too small
- not as secure as top-end u-locks
- more expensive than less-good locks
Here are the trade-offs between smaller and bigger u-locks:
smaller bigger lighter heavier less convenient more convenient more secure less secure
When locking with a u-lock, you want a snug lock-up. Longer u-locks may be more convenient, but they’re much easier to break with a leverage attack (pipe thru the lock, twisting). The Futura 64 sometimes is too narrow to fit around some thicker objects like parking meters, but I’ve never been in a situation where I couldn’t find something to lock to.
I would call this a “high-end” lock, but it’s not tip-top. If you live in a seriously theft-prone area, maybe you need a Kryptonite New York Fahgettaboudit Mini Lock. (I don’t know if there is anything else as burly as that.) But for the vast majority of cyclists, the Fahgettaboudit is overkill. To my mind, the Granit Futura 64 is the sweet spot in terms of security per dollar, and security per pound.
My second choice option is the Kryptonite Evolution Mini 5. This is very similarly sized to the Abus, above, but is a little wider, increasing lockup options. The Mini 5 lock does have a very good bicycle mounting system included. The downsides are that the Kryptonite is heavier than the Abus, and locks on one side of the U with a bent foot on the other side. Theoretically, this makes it less secure than the Abus.
I don't really have any way to judge which of these has the stronger steel, but from what I've read online, it sounds like the Abus has better quality steel. Will it make any real difference? I don't know.
Really, the difference between these higher quality u locks is probably not really significant to make a difference in your bike being stolen. There are other similar options from other brands which are probably just as good, for about the same price.
The important thing is to not buy a cheap u lock. Every brand has a range of u locks available (including Abus and Kryptonite). Deciding which u lock to buy based on brand is the biggest, easiest mistake to make. You really need to be looking for the most secure lock you can stand to take with you (having a smaller lock helps on this), and that you can afford. Buying a $25 u lock is probably a waste of money.