Best RPG graphics on the SNES... hmm that's pretty tough to say.
Art work in many RPG's is better to me then the new everything has to be as close to "photorealism" as possible.
I think the following RPG's had a great art style
Breath of Fire I and especially II for brilliant character models, animation, underwater scenery, backgrounds. monster builds, and awesome bosses. Where as the series in whole may not have the best graphics bit by bit it definately holds the most charm and in my opinion non forgetable sprites and scenery.
Honorable mentions go to
Lufia I and II= These games have an art style kind of like BoF only not quite as good. However, some could argue that the HUGE boss battles were captured perfectly!
Final Fantasy II and III= beautiful backgrounds
Super Mario RPG= lovable Mario characters with great graphics... however the downfall in the graphics department was most of the backgrounds although some were absolutely brilliant. My overall honors could not go to such a short game however.
I think its probably chrono trigger from an objective point of view but you could just say whatever game you liked the most. I think chrono trigger had the best drawn highest quality 2d rpg graphics on snes. Tales of phantasia though was pretty good too.