Cruise Critic considers Viking a luxury line. This is what they wrote about it:
About Viking Ocean Cruises
Viking Ocean Cruises is one of the youngest luxury fleets on the waves, with beautiful, largely inclusive 930-passenger ships that generally cater to adults 55 and older. Itineraries are packed with ports, and a shore excursion is included at every stop.
They are small ships and every guest room has a veranda. They look very nice! Maybe you don't think of them as a luxury line because of their mass market river cruises.
I voted for Regent. I am booked on Regent, but my only other experience is on Princess. I am interested in Crystal but not so sure because they are not sailing yet. I don't really care about a cruise line being labeled luxury but I also don't think Crystal can be labeled until after they start sailing again.