Viking is a premium line, offering food and amenities that are a step above the main lines. We have sailed with Princess, HA, X multiple times and enjoy, but Oceania and Viking are our current first choices. Star is a lovely ship, very clean and well maintained.
Lots of folks talk about heated floor in the bathroom .. sounds like a minor thing, but it is really nice and makes a difference! This attention to details is what is great about Viking.
Overall, we like the food and restaurant selections a bit better on Oceania, but Viking in MDR was very good, and many meals were excellent. We generally prefer formal dining at large tables on Princess and HA .. Viking is personal choice for dining, but tables for two are close to each other and friendly passengers make the experience similar and nice. We found "included" wine selection to be good enough. Included wines rotate each night to pair with menu, but we were always able to also get one of the choices from early in cruise that we liked - just ask....
Dubrovnik is highly interesting history .. worth the time for a tour