My favorite stop was Krakow, Poland. I wish I'd seen more of Poland, but this was the only stop I made there. It's a really beautiful city with architecture from almost every century, and it's near Auschwitz which was the best and most life changing tour I took (for obvious reasons). Krakow is wonderful just by itself because there's a lot of World War II history in the city and it has one of the largest town squares in Europe, as well as lots of museums. Plus, Poland doesn't yet use the Euro and the exchange rate is excellent against the US Dollar, so you get a lot of bang for your buck. I learned A LOT on my trip and plan on writing some more hubs, so check them out in the coming weeks.
The best thing I learned about travelling was how to be flexible and to always go with your gut. Krakow wasn't on my original itinerary but I added it later and it ended up being my favorite stop. Also, it's good to check out the cities that aren't so big or central because you get a better cultural understanding and travelling off season lets you escape the big tourist groups. Also, staying relaxed and flexible helps avoid a lot of culturally based frustrations you may experience.
Overall, I learned a lot about history (especially World War II history) that I was never taught in school. It overall just made me a more well-rounded person.