I found this on the Travel and Leisure Web Site. A survey of readers for 2013. There are a bunch of categories. Here is the link for River Cruise Companies for 2013. It also shows the 2012 rank.
Travel and Leisure looks like it is a travel magazine published by American Express Publishing Corporation. Looks like they also publish Food & Wine Magazine and a few others.
2013 World's Best Cruises + more | Travel + Leisure
Here is the Conde Nast Travel survey results for 2013
Condé Nast Traveler // Gold List // Cruise Lines // River Cruises