[chrisrogers]

I'd use a large chain and the beefiest lock you can manage. The sort of thing you'd use to lock up a motorcycle.

[zimpenfish]

Same suggestion but also multiple locks - I always use 3 (D, chain, metal bar) in London and I've never had a problem, even with occasional overnight leavings. If the bike next to yours only has one, you're already ahead in the risk calculation.

[heynowheynow]

37/80 + 14KS. It's a tight fit, heavy, and expensive but they work.