My basis of judging a good RPG is based mostly on gameplay and replay value. While I don't care much for the storyline of a game, a terrible storyline can still destroy the play value of a game just as an excellent storyline can make up for some of its minor faults. Based on these two statements, I think I can fairly say that FFX-2 is my favorite Final Fantasy. Of course, I've only played VII, Tactics, VIII, X and X-2. Perhaps some of the earlier ones as well, but not enough to rate them.
My first experience with this game was, unfortunately, watching one of the really bad singing and dancing scenes when I was at school. This put me off in terms of how much I wanted to play the game until a friend personally recommended it as his second favorite (after VII). Eventually, I rented it and played for a few days - I loved it. The storyline is not so hot. It's essentially a game made up of sidequests and a VERY short main storyline. It's doubtful that you could beat the main quests and only the main quests without leveling elsewhere but if you could, it might be a 5 hour game. The key here is that every chapter (there are five total) has specific scenes and sidequests that you can undertake at 15 different locations. If you don't understand the dynamic of how the sidequests work (if you move on to the next chapter without completing a sidequest you're often unable to complete it and that effects your game completion percentage) you may end up, like me, restarting the game after 10 hours of play. Regardless of if you feel the need to get 100% completion in the game, I think the feature that makes the game so appealing to me is the skill system. Let's back up just a moment and explain these things called dresspheres - rather than getting different characters all throughout the game as is common in the FF series, you get different dresspheres throughout the game and these allow you to use different jobs. Yuna's default job is Gunner and she has a skillset that is specific to that job. Skills within jobs are controled in an AP-like system where you get a certain amount of points for killing, being attacked, using items, etc. and these are dumped in to a skill that you have selected to focus on. Skill point requirements range from only 10 or 20 points for simple skills all the way up to 300 for some really great abilities. This system is similar to the skill distribution in Asheron's Call which is easily my favorite MMORPG and I think the item that ties them together and makes a good RPG for me is this skill system that's not based on levels.