2 Locks.....
Good D-lock - Gold standard, and a gold standard chain.
My bike is in a public access area all day, so uses two locks. The D-lock stays with me - don't forget how you lock your bike - with D-locks you need to fill the D with as much bike and stand as you can so they can't get a bottle jack in - Locking through the rear triangle, or using a small D will help here.
I also use a 10lb chain lock, but that stays at work as it's too heavy to carry.