I use a hardened steel chain and a hardened steel lock, and I thread the chain through the frame and the front tire. That’s enough to defeat bolt cutters (and my lock has notches on it to prove it), though I’m not sure how it would do against an angle grinder. Though if they have an angle grinder, they might just go through whatever it’s locked to instead of the chain/lock itself. There’s only so much you can do against a very determined thief.