[font color=\'#000000\']Where to start?!? Europe is a varied place; what are you most interested in? History, architecture, outdoor scenery, quaint atmosphere, busy cities, quiet countryside, museums, beach resorts, nightlife,...? Your own interests would determine where best to go. If you give me more data, I can start to make suggestions. If your primary interest is photography, then I'd suggest you browse photos on web sites, in book stores, etc., and see what areas look most interesting-to-photograph to you.
Finding a group to travel with where you stay in one region for three weeks may be tough; most tours try to cram in as much as they can get away with, and are nearly useless except to get a brief (and tiring) overview of a large region. If you know a small private group of people to travel with, that would be much better. If you don't, you might be better off finding out which countries are the easiest for foreign travelers and going yourself; the more tourist-friendly countries are really not difficult at all if you read a couple of good guidebooks ahead of time so you know what to expect. If you already know a little of a foreign language, that will make some places more pleasant (for instance, France is a frustrating place to visit if you don't know any French), but in many other countries a large fraction of the people you meet will know English (even in Britain ;-) ). Also, some European countries are much more expensive than others - if cost is important to you, you should take that into account.
If you go during peak tourist season, you might need to get reservations ahead of time in many places. It will be easier to go off-season and "play it by ear" regarding exactly where to stay and when you want to move on to the next place, not to mention things will be less crowded with tourists.