A note before recommendations: all bike locks can be defeated either by picking or destruction. The most important function is to slow the theif enough to make it not worth their time.
That being said, Kyrptonite makes a good line of bike locks.
Heavy chain (with a sleeve to protect your paint) and a sturdy key lock.
Stay away from barrel style key locks.
My recommendation is to get something that takes more that a normal pair of bolt cutters to defeat (think more than a 1/2in chain). You will never have one large enough to beat a grinder, but you can make it difficult for them.
The more secure the lock the heavier it is going to be, so that’s a thing to keep in mind if weight is an issue.
Yes, but most of the are not practical for bikes, or are prohibitively expensive ime.