U lock with a galvanized steel pipe T over the spot where the bar received the U. That way the crooks cannot put a lead pipe over the end where the key goes in and bend that and snap the lock. Heavy but it works. Put it in backpack.

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They constantly try to escape from the darkness outside and within
Dreaming of systems so perfect that no one will need to be good T.S. Eliot

Not this one.

I think chains are probably better than U Locks, but nothing is going to deter a determined thief.

Here's the lockpickinglawyer picking the chain and lock he uses in under a minute.

The Abus Bordo line is great. Super solid, hardened steel, but folds up much smaller than a U-lock. They can even do multiple locks for you with the same cylinder so that you don't have to have multiple keys for your bikes.

That lockpickinglawyer makes short work any lock and I haven't yet seen a bicycle lock that he would recommend!

I'd hat to admit it, but I bought an ottolock thinking that it could work for short coffee stops, only to regret my decision after the lockpickinglawyer showed how he could cut the lock with hand-held tins snips in one cut. Looks like the ottolock is about as good as zip-ties:(.

Another vote for U-Lock ... Kryptonite New York FTW. Cables & most chains are trivial for a determined thief.

Certainly a beefy lock, and it will probably make a casual thief move on to another bike. Takes him about 1:30 to break it with a nut splitter.