In the US, I have to say prolly NYC is the biggest and best. DC's Metro and Boston's T are close runner ups. BART is also ok, no issues with them. Matter of fact, I use them frequently, because my audiologist is in Oakland, and I'd rather not drive and park there.

As to Europe, DeutschBahn is pretty hard to beat. I feel as if I know the Rome Metro as good as my own driveway lol, and yes, BKK's MRT is quite teh awesome to cool down, but as said above, limited locations. Oh btw, Dubai's Metro is pretty awesome too!

Almost forgot about HK's MTA.....took me most everywhere I wanted to go.