It's been a while but I've done a fair bit of busking in my past. Here's what I know:
London's tough. On the underground you'll get moved along pretty quick before you get a chance to make a lot of money. One of the markets could be a better way to go. Bye laws could stump you though.
Copenhagen has a great pedestrian st (called Stroget) that can have a great busker scene on it. I've started playing there on my own to suddenly find myself amongst an impromptu band of diverse musicians. Great fun! You can make a lot on a sunny Saturday afternoon if you're good. No amplified instruments as far as I remember.
Barcelona is great like someone mentioned. La Ramblas is an amazing place to play but the competition can pretty hard. There are some very, very good acts there who make their living on street performing. Maybe look a little into the squares in the Barrio Gothic instead.
Paris is great if you get the right spot but again there is a lot of stiff competition as it too attracts the greatest in Europe.
If you get a chance to hop the water from Spain to Morrocco performing in the main square of Marrakesh, Djeema El Fnar, is something you'll never forget. Although I only made money in Essouira, a far smaller town on the coast.
Dublin is great as it's always busy, there's a decent pedestrian area, people are always up for good music. You can clean up on a good day. You'll also meet a lot of people. We're a friendly bunch.
If you're music doesn't naturally make the transition to the streets you should re-arrange things. I've always found that originals or well worked covers with a difference are what works best. Quietly strumming No Woman No Cry just ain't going to cut the mustard. Maybe I'm telling you something you already know but busking is a lot harder to do well than it may seem. It can be tough. It also changes how you sing. After my first summer of busking I went into the studio, as I was warming up the engineer called me in from the live room and asked me to record in the machine room, I asked why he said I was singing so loud that he reckoned all I would have to do is sing in the direction of the tape machine and it would record it.
It took quite a while to undo that habit. I think it's a rite of passage every singer/songwriter/musician should go through. It's been part of the tradition of music longer multi track recording. There's a lot to learn from it and a whole lot of fun to be had. And did I mention the chicks?