We have time only for 2 maybe 3 cities..

I spent an afternoon in Nürnberg. Visited the castle and the public market. There are some Nazi sites there but I didn't visit them.

Rothenburg ob der Tauber is an easy day trip from Nürnberg and you can do it with TagesTicket Plus(all day rail pass) from the Nürnberg metro (VGN).

I particularly like Marienberg Fortress in Würzburg; the Residenz, not so much.

Only been through Regensburg on the train; no opinion but some say they liked it.

Bamberg is also nearby.

I lived in Nuernberg for 3 years, so I'm going to vote for it as your central location. It's a lovely city with lots of history to see on its own, and depending on how much time you have to spend, might keep you occupied the whole time. If you do that, eat some Nuernberger Bratwuerste and Schweineschaeufele for me!

I went to Rothenburg ODT for fun and Wuerzburg for work-related activities. I don't recall ever going to Regensburg. So, if I was going to pick among the 4, I'd pick Nuernberg and Rothenburg ODT. If you haven't already, one approach to researching them is to do a Google images search for the cities for an idea of what you will see. By clicking on some of the pictures you may be lead to good sources for more information.

If you haven't already, you can go the the DB Bahn search site (http://reiseauskunft.bahn.de/bin/query.exe/en) to find out the details for any trips from there to the other cities. Be sure to click on the map for the route. Also be sure you select Rothenburg ob der Tauber from the drop down list when you start to type the destination. By the way, depending on the train you take, it's about 1.25 hours each way. Wuerzburg is about an hour each direction as is Regensburg. But you may have already checked these things out.

When Robyn uses the Bahn website, she should be sure to type in Rothenburg, with a 'u'. The Bahn will recognize Rothenberg as a legitimate name and the drop down list will show a lot of places with "Rothenberg" in the name, but not "Rothenburg ob der Tauber".

"Nuremberg" and "Wurzburg" should work in most cases for Nürnberg and Würzburg, but if you can't make the umlaut, Nuernberg and Wuerzburg are grammatically acceptable and will probably get more reliable results.

I agree with Lo. Not having been to Regensburg I cant personally say if its good but I know the other three are worth the effort. If you don't have much time, Nuremburg would be a good place to base yourself. Otherwise, try to stay perhaps 1-2 nights in Rothenburg and 2 nights in Wurzburg.