Hardened chains of sufficient thickness can stand up to an angle grinder pretty well, to the point where thieves will rather steal another bike because angle grinding for that long will attract attention.

Ring locks suck, a lot of them can be defeated with a pair of scissors. Similarly, U-locks suck because they're never as strong as the bike frame. You can just pick up the bike and use the frame as lever and the streetlight pole as fulcrum, twisting the bike around until the locking notches of the U-lock snap.

Occasionally, in The Netherlands professional bike thieves will drive up with a stolen van at night and load up entire bike racks. Not much you can do against that except store your bike inside.