We do actually have some information on Rivain. We know it's:
- matriarchal
- not Andrastian
- primarily non-white
- not anti-mage, and in fact, allows mages a good deal more freedom than most places (excluding Tevinter)
- not anti-elf
- not anti-Qunari, and is majority Qunari (as in viddathari) in Kont-aar
- tight-knit and values community
It's not perfect, but it sounds way more interesting than any of the other nations, imo.
That's a generalization. What we know is that Rivain is not a monolithic society. The country is very unstable, one step more and it will become a failed state. It has been the scenario of one of the few documented cases of genocide in Thedas (almost causing another Qunari War), it's a refuge for pirates and raiders, and there are tensions between the Andrastian upper-class and the traditional populace. The leaders of the kingdom are actually Andrastian, anti-Qunari and last time we heard of the country Seekers and Templars had just purged the mages in Dairsmuid, effectively putting an end to the compromise they had with the seers.
If you were talking about "best" in the sense of "most interesting", I may agree (I have a soft spot for Tevinter in that regard). However, if we're talking about "most decent", I have to agree with TKS. Ferelden is so far the most decent country in Thedas, as far as we know. And Ferelden is not a nice place, so it speaks ill of Thedas in general.