TLDR: Mass Effect. Mass Effect had a positive impact on me because its side quests were so stupid. Weird but true.
Long version:
Drilling down into a question like this is difficult. The mark of a good RPG is not how it affects me but how well I can affect it; how well I can shape the experience mechanically to satisfy my imagination.
So am I looking for an RPG with, say, a strong, emotional story? Because that will tend to be less of an RPG in my mind, not about the story I wanted to tell. Like I remember a particular plot point in KOTOR 1 that was a very affecting moment, despite being nothing but a cutscene reveal that had nothing to do with my choices as a player. Is that what we're looking for? Does that count as "affecting me as a person"? It was a stunning scene but it didn't change me.
I guess I could just say what's the first CRPG I ever played (Maybe Wasteland? Wizard's Crown? that can't be right), because obviously whichever game that was started me down a road. But I don't know. I was already a video gamer and I was already a tabletop RPG fan, so while mixing the two was an important moment, which specific game it was is a mere detail.
Maybe I'll look at the game that had the most substantial effect on my behavior. Weirdly, the one that comes strongest to mind is the first Mass Effect. Maybe the game changed me, or maybe I was changing at the time and happened to be playing that game. Either way, I remember that there was a strong, motivating central mission, trimmed on all sides by plentiful and (relatively) meaningless side quests. Thanks to this imbalance, Mass Effect became the first game to substantially alleviate my completionist "Achiever-type" compulsions. Although Saren and the Reapers would have patiently waited for me forever before destroying the galaxy, I declined to solve less urgent problems for many random passersby. Some guy couldn't get the government to release his dead wife's body to him. "Not my problem, I have a galaxy to save." Some woman in a bar was being blackmailed into continuing to work there. "Not my problem, I have a galaxy to save!" The game was no longer a checklist of obligations, but instead the story I chose to tell.
Was it a video game that (inadvertently) taught me to let go of "sunk costs", that it didn't make sense to clean my plate if I had no appetite left? More likely I was maturing as a person and my approach to the game reflected that. But I'll always remember my reaction to that guy talking about his wife: it doesn't make sense for me to accept responsibility for this ... so I simply won't. Very liberating moment.
