I was not a huge fan of Rorona. Totori was leaps and bounds better, like Kevadu said, while Meruru's improvements are on a smaller scale. I really think it's the best in the series, but I can see why some people might like Totori a bit more. Remember that there's a prologue in Meruru that will succinctly summarize the first two games.
Also, there is an event in Meruru that spoils the true ending on Totori, so be forewarned.