I'd say that you want a good running older bike that looks like crows have been gnawing on it. Find a suitable bike that was in the mid to upper range way back and get a bike shop to rebuild it. It'll cost you but the bike will ride superbly and with the "crow gnawed" look hopefully it won't draw a lot of attention.
Just get one that has fender and rack mounting eyelets on it and then set it up with some really horid looking fenders and a rack. A little rubber ducky honker that warns pedestrians of your approach will further chase away any thief with a shred of self respect.
If it's not gnawed enough looking then do a little disfiguring of your own. Then just wax the damaged areas with a good car wax to resist any rusting. Or rub on some boiled linseed oil to cover it up so it won't rust but still looks like heck.
Two of my best riding bikes are old "crow gnawed" bikes from the late 80's for one and mid 90's for the other. They really knew how to make some excellent steel frames back then. Look for an older high end mountain bike since any of the early road bikes are going to be targets for the fixie crowd suppliers. Besides, an old high end rigid fork mountain bike would make an excellent short hop commuter and errand getter thanks to the more upright seating position. Just stick some platform pedals on it and enjoy the ride at a little slower pace.