Earthbound (Mother 2), because:
1. It was the first RPG I can remember that didn't take place in a fantasy or pseudo-fantasy setting.
2. It had a genuinely fun battle system.
3. It was truly and genuinely funny, only the various Mario-related RPGs can say that.
4. It has a huge cult following, since it never had any other games released in America.
5. It has a good story despite (or perhaps because of) the fact that it doesn't take itself seriously.
6. Many of the people that worked on Earthbound went on to work on the Pokemon series.
I have to agree with this guy. I loved everything about this game, for once, I wasn't playing as a stereotypical class or race, or use the generic sword, lance, or any other generic weapon to attack enemies. Instead, it was just a normal kid with some really weird friends going on an adventure to defeat pure evil(Giygas, another awesome character).
Not only that, it has one of my favorite soundtracks to ever appear in a video game. For example, just listen to the theme that plays just before the main menu, it's just nostalgic to say the least. The main battle theme was just weird, and unlike any music anyone ever heard in a game before, which is a good thing. It was innovative, and even had some psychedelic/progressive sounds to it, along with jazz sounds to it. By the way, the jazzy tune to Fourside was kickass, before they decided to **** it up in Melee.
It had a clever script and solid characters. I'm really looking forward to the Mother 3 translation and I hope it is half as awesome as Mother 2(Earthbound) was.