IV is absolutely my favorite, I played it back when it was II.
At the time I was a mostly arcade gamer, fighting games, racers, shooters, etc. Buddy of mine got an SNES and this game for Christmas, and he literally would not stop talking about it, so I agreed to play the intro to shut him up.
Two hours later, he's like "So are you done?" and I go "Leave me alone, this is awesome!" to his general amusement. The concept of a game that had an actual plot, characters who interacted with each other, the scale of the world, it was a sea change in my gaming life, and RPGs have remained my favorite genre since.
I don't know what my first RPG was but I think FF6 was the first one I really played.