Mark, I don't have a rec for a bike lock, at least not yet, altho' I am considering one option. Two suggestions for you: 1) Visit a locksmith and discuss padlock choices. When picking a lock system - the ABUS lock mentioned above is a good example - the manufacturer is choosing the actual lock, so we have no choice in how secure it is/isn't. And normally the manufacturer doesn't indicate e.g. how many pins are used in a keyed lock, while we know the more the better. An experienced locksmith should have a more critical view on the relative security of various padlocks, which can then be added to a length of proof coil or HT chain that's slipped inside an old inner tube. So far, that strikes me as secure an option as any, altho' unfortunately heavy & bulky. 2) Type 'lock picking austin tx' into youtube and watch a few of the videos done by Scam School. Austin has a 'lock picking club' and several members demonstrate their acquired 'skills' in the videos. It will give you a good feel for how locks work, how easy some are to defeat, and why some are better than others.
Back in the day, one of my collateral duties on a sub was Top Secret Control Officer. I was solely responsible for all the crypto cards, TS intel summaries and so forth. Scared the hell out of me, since a former shipmate I knew had lost a batch of crypto cards on another boat and the Navy made him deeply regret his error. Everything was stored in a very old safe and, as were were preparing to leave the shipyard after an overhaul, I discovered one day I couldn't open up the safe. I talked with the crypto guys at the yard and they sent down the shipyard's locksmith to fix the combo lock. So I escort him to the safe, he puts his little bag of tools down and tells me to leave the compartment. I explain I can't; I'm solely responsible for all the safe's contents and some of them are highly classified. He shrugs and says 'You leave and I'll fix the safe. You stay and I'll leave.' So I step outside the compartment, he pulls the curtain closed, and in less than a minute the safe is open and curtain pulled back. Locksmiths, it turns out, know stuff we mere mortals do not.