A big ****-off dlock is almost always the best portable option. But they’re bulky and not always suitable for locking to everything you’re locking to. But you can’t really carry a useful chain that’ll actually slow down anyone with the right kit, and so once you give up on that, you’re basically dealing with opportunists and idiots so you only need an opportunist/idiot proof lock, there’s no real middle ground.
I really like the Onguard Brute d-lock. It’ll beat almost any non-hydraulic cutters, last as long as anything against a grinder, and they’re very cheap. The build quality is poor so they do corrode, if left exposed outside and the lock unfortunately will jam eventually.But if you can keep it dry or regularly lube the lock barrel it’ll do OK. It’s barely more heavy and inconvenient than most lighter duty d-locks and better than most expensive ones.
(they do a long and short, tbh the short one is usefully small though still heavy, but it’s just too small to be guaranteed to lock the bike to anything but a proper bike lock or thin fence, so that’s a bit limiting)