Hmmm...I'm not sure, to be honest. The first game that springs to mind is Tales of Symphonia, since it was my introduction to the JRPG genre...well, I'd played FFVII and FFVIII beforehand, but at the time I wasn't really aware they were JRPGs; they were just games I'd played and thought were alright. ToS had an anime-esque style and a far more captivating story and cast of characters, helped in no small part by the voice acting, and I found the battle system more engaging. It just seemed to click with me more than FF did, and encouraged me to try more of the genre.
That said, I'm very fond of Golden Sun. I remember being extremely excited about it, then finally getting it along with a black GBA, and I loved it. The Lost Age was the better, longer game, but the original has particular significance for me; it was just so much fun. Taking on Saturos at Mercury Lighthouse, summoning Thor/Meteor/Judgement for the first time, taking out Deadbeard after slogging through Crossbone Isle, and then finding out that there was going to be more to come...yeah, Golden Sun is a definite competitor for this.