My European experience is limited, but I plan to travel more as soon as grad school is done (in 2 weeks!!!!)
Ireland - Been twice, love it. Want to buy a house there and rent it until I can retire there permanently. Seen most of the whole country.
UK - London is huge and driving through the countryside was so fun. Also went to Lancaster, Liverpool, and Windsor on the same trip.
St. Petersburg, Russia - Just got back a few weeks ago. amazing trip and GORGEOUS women. Have to go back when it's warmer out.
Frankfurt - Well, only the airport, on a stop-over to Russia
We plan to go to either Spain, Italy or Greece in the next 10 months.