The London tube is very good for consistently being able to get you where you're going. In many cases, if you miss your train, there's another one coming in <10 minutes (during the day). Clean trains, too. And you will never, ever, ever be able to forget to "Mind the gap". The tube system is well integrated with teh busses, as well. Although, from the time I was there, if you're going anywhere inside London proper in daylight hours, take the tube. Surface streets are a nightmare :).
Chicago's El system is also pretty good. Although, being spoiled by Metro, I was most disturbed when I discovered you can't call them and ask "What trains/busses do I take to get from here to there by this time?". The lady I spoke to was very nice about it, but explained that she didn't normally try this, so she wasn't sure that she was giving me correct information.