If it's for leaving outside at night, then a 19mm thick chain for (leaving permanently) outside your flat, AND a u-lock for portable use.
Or you could think differently and get a foldable bike, like a Moulton? If it's in a case, and collapsed, it not a bike anymore, it's luggage.
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If it's for leaving outside at night, then a 19mm thick chain for (leaving permanently) outside your flat, AND a u-lock for portable use.
Or you could think differently and get a foldable bike, like a Moulton? If it's in a case, and collapsed, it not a bike anymore, it's luggage.