Here are of the logos of the Metros that I have traveled in so far.
The easiest to figure out was Delhi followed by Milan (with a little help from Muzayun). I tried Delhi while I was still in TVS and was damn impressed with it all then, as I am even now. With Milan Metro also I got almost instantly comfortable. And I did travel alone a few times there, even to unknown destinations. We often also bought just one ticket and hence managed to travel for half the price!
Rome and Milan Metros share the same logo, but that's where the similarities end. Rome Metro is almost dirty, and full of graffiti art. The life-on-the-move that one sees there isn't sanitized like Milan, but that doesn't still necessarily qualify as 'good'.
Even Istanbul Metro wasn't very complicated and the Metro was surprisingly clean. Of course I missed my flight out of the city admiring the place and the damn Metro! An 'Akbil' gives you access to Metro, Tram etc. for one trip (what ever the distance).
Kolkata was a bit confusing (apart from being almost like a Mumbai local) as I just went in with no map of the city in my hand or mind. It took me a while to understand that I was traveling north after I got into the train. My travel was completely purposeless there, so there was no fear of reaching the wrong destination. And of course Kolkata was also the cheapest.
Paris Metro was simply mind-boggling but the most impressive of them all. It wasn't difficult to get lost in the different underground levels, plus it was also perhaps the most expensive. The Metro stations in Paris are also full of life, music and its almost like a cultural melting pot. There is tremendous scope for entertainment for me during my next visit to Paris at all these Metro stations.