Whats the best bike lock to use?

Well don’t use a U-lock, as you can undo them with a ballpoint pen. I generally use a length of chain long enough to loop through both tires and the frame, and a strong lock, generally I would say masterlock.

I would recommend the Kryptonite New York line. There are several to chose from.
Example- https://www.kryptonitelock.com/products/ProductDetail.aspx?cid=1001&scid=1002&pid=1242

I like the number lock ones. Don’t know the name of them though.

Look here, hope it works! :-/

http://www.bikeradar.com/gear/article/buyers-guide-to-bike-locks-20408

Combination locks can be easily cracked, so im not really sure what the lock should be, but a chain to loops through both wheels and fram is a must.

The ballpoint pen trick is no longer true.

The vulnerable locks had tubular cylinders and they were phased out 5 years ago. The cylinder and key looked liked this , if you have one, get rid of it.

Kryptonite, the most popular u-lock, replaced all tubular cylinder locks for free. I traded in a 10-year old lock, they paid postage both ways.

Here’s a good guide to bike locks:
http://www.missinglink.org/Pages/bike_locking

U-Locks can be busted with canned refrigerant and a hammer, but they’re the best theft deterrent. Cables and chains can be cut with bolt cutters, the kind bike thieves always have on them. Personally, I was never enamored with U-locks, as they’re inconvenient to use and store on the bike. I have a cheap little combination cable lock that I use when I want to go into a 7–11 for a couple of minutes. Otherwise, I take the bike in with me. When I ride to work, I bring my bike into the building. If your boss won’t let you store it inside, keep your nice bike at home and commute on a sub-Huffy.

Depending how urban the area, just don’t leave a nice bike locked up. It should be in your office or whatever. A Ulock through the wheel and frame is a nice touch to prevent anyone from riding it away, but use a REALLY thick coated chain and a solid padlock to lock the bike up. Run the chain through the wheels, the frame, and whatever you’re attaching the bike to. Combine both the U-lock and the standard lock/chain (which unfortunately will involve a backpack or briefcase to hold all the gear) and you should be golden in most areas. Again, I repeat, ‘MOST’.