8-bit era (mid to late 80s): Probably Dragon Quest. It came free to many people who subscribed to Nintendo Power and was the first RPG for many people. It was also the inspiration for both its own series as well as FF and many others.
16-bit era (late 80s to early 90s): Chrono Trigger. I'm not sure how much it influenced the genre but it's arguably the best RPG ever made in any era.
PS1 era (mid 90s): Final Fantasy VII. I don't think this one needs explanation.
PS2 era (early 2000s): World of Warcraft. Not even a PS2 game but that's just how I describe the era. Anyway, clearly the most popular MMORPG of all time and a clear influence on both MMOs and single player games released after.
PS3 era (late 2000s): Mass Effect. I think this was the biggest influence on the western action/RPG/shooter hybrid which now most popular RPGs fall under this category. Also built on character interaction and story choice that was pioneered in other games but really popularized in this series.
PS4 era (2010s): At this point I would have to say Breath of the Wild. I mean, at what point are we going to consider Zelda an RPG? I could see a finer line in the past when action and RPG were more distinct but it's really blurred now. BotW seems to be the magnum opus of this series that finally takes it to the next level. However Witcher III and Persona 5 (return of the JRPG?) are strong contenders.
I'm sure there is an Elder Scrolls or Fallout game that goes in there somewhere but I could never get into those.