The classic U-lock comes to mind naturally when we talk about (motorized and non-motorized) two-wheelers. There are a few good reasons for that. After all, it doesn't become a classic just like that.
It is a solid rod of steel, using a sturdy locking mechanism and because of its shape it is also relatively easy to carry and use. Material and technology are specifically designed to remain as resistant as possible to any attempt at theft.
We would like to point out that everything can be broken. If necessary, combine a U-lock with a chain at an anchor point. In any case: the more harder the challenge, the fewer thieves will attempt it. Combining different locks is always a good idea.