We have never been to this part of Europe and enjoy visiting historical and UNESCO sites.
We like Viking because it's a "grown up" cruise, no children, no smoking on board, no casino or other money making ventures. They offer very good included tours of each port city as well as additional tours that you can book. The service on the ship is like a four star+ hotel; nice friendly staff who make an effort to get to know you by name from the room stewards to the dining room wait staff and kitchen staff. The food options are wonderful. You can choose full breakfast, lunch and dinner daily or buffet style which was great for a picky eater like me. We also enjoyed the locally inspired dishes that were available every night. Viking also offers good entertainment and lectures about the locales being visited.
The only thing we could recommend would be a guest accessible laundry - coin op or scan with shipboard card. If we cruise again, we would like to pack less and wash more.