It really depends on the time of year.
For the UK this time of year for example, i'd really only recommend London. If you like street photography, it's as good as any city in the world - but bear in mind the weather will be pitiful. Cold, raining, etc - expect it all. I would definitely not put the rest of the UK anywhere near the top of the list for winter / photographs tho. But London is a big thumbs up. Having said that, if it's in the summer - there are some beautiful parts of the UK all very easily accessible. Scotland is just amazing for landscapes for example and can be reached by train / car or v v cheap flights on ryanair (like < 10 ukp). You can get to london from Paris via the eurostar (train) from Paris. V easy, 3 hrs and you'll arrive in central london. I'm not aware that you'd need a visa if you hold a Singaporean passport.
Paris is truely amazing all year round. The world's best street photography took place in that city. As with all cities in Europe, taking photography at night, I'd usually recommend making sure you watch out for your belongings, etc. My colleague is from Normandy - says it's v beautiful but again, depends on the weather. This time of year is UK-like, whereas summer as awesome. One place I have always wanted to visit but not have the chance yet is St Michael (someone else's pic here -
http://www.pbase.com/eldarrio/image/54153601). If you're interested, I can ask more abt it. S.France is fanatastic year round. Some beautiful places around Nice, etc. Small artists villages, etc.
For Italy / Greece / Spain - all amazing countries. Truely beautiful countries and great mix of beautiful countryside and old historical cities. Any search at any of the major cities or regions will produce amazing pictures of landscapes, old european architecture and so forth. Can't go wrong with any of them imo.
Central Europe offers some v interesting places like Prague, Bratislava and so on. All v nice, v beautiful. Quite different from western europe.
Amsterdam - fantastic place to visit to have fun. Not sure about photography further afield. I imagine it's v nice but i've never ventured out of the city.
Scandinavia I would personally avoid due to the weather and time of year. There are far better times to go than winter when there is only a handful of hours of daylight. During the summer, it's the opposite - light until 11pm in Oslo, Norway for example. Haven't shot there tho - it doesn't tend to be as big a pull for photographers.
Vienna - one of my favourite cities in Europe. Truely stunning architecture, some of the best museum's / galleries in Europe (home of many famous paintings from the secession). Great waether in summer and bitterly cold in winter as with the rest of Western Europe.
You know what... all this email is doing is reminding me the diversity and size of Europe. I think it impossible to really recommend any countries above the other on purely merit... all so different and depending on what you want to shoot. So long as you arrange certain things in advance, keep safe and are flexible, i think you can't go wrong
. If you want to know more about a specific country however, i'll be happy to provide more info if i happen to have been there.