Viking River Cruises prides itself on top level food and service, which I can vouch for. The cruise is for English-speaking passengers ....and I'm not sure about
Uniworld's boats. This is important to check out because of tours -- you certainly want them in one language. And a lengthy trip works best when you're all one nationality.
I don't personnally know Uniglobe, but I can say that Viking is very, very good.