London Underground was the first major subway system I really spent a lot of time on. (I'd used the Chicago CTA a few times as a kid, but not enough to really know it.) I always felt like London was very easy to understand for a non-native.

New York, OTOH, seems damn near incomprehensible - especially if you're there on a weekend when they're doing a lot of track work. I had an easier time figuring out the Tokyo system with zero Japanese language skills, and that isn't the easiest system in the world to understand.

Most of the European systems I've used that I can recall are good. Paris, Moscow, Munich, etc. Well-marked, efficient, and maps that are understandable even with limited or no local language skills...