Paper mario by far, sure there are some that out do it, but for me it was the best.
Mainly because it got me into rpgs to begin with, plus it had nice battle customization.
Not only was it in level ups but later on the badge system really shined. IT allowed you to have a very nice play style of your mario and his companions, offering all kinds of way to battle from getting low on hp to do massive damage, to be a destructive tank, from relying on other defensive tactics such as the volt badge, or to surpass spiked foes with a certain badge to hitting almost anything with a hammer via hammer throw or quake hammer.
So many options for the gameplay. Then theres the fact of action commands, while not having the greatest variety ever, it was still enough to be called unique compared to so many other turn based rpgs. From hitting the button at the required time, to using the joystick to attack, or maybe even holding a button for a certain amount of time to do more damage.
There was a lot in store for many gameplay options, even badge attacks had their own action commands.
I also liked the game because it showed some side views of characters that normally you wouldnt see in the other games.
Such as luigi's diary or the chapter end with peach trying to help out mario, even bowser had his own personallity that made him far more likeable then he ever was.
There were even several new characters such as goombario, general guy, koopa bros, and even watt. Still many more to add to that list but yeah.
The only flaw that i see in the game even to this day is the sidequests, as most of them were fetch quests and i dont really like fetch quests.
Overall though paper mario is fantastic to me, ive played 30 different playthroughs and even 100% the game on several occasions.
A top of the rate rpg and even to this day it remains as one of my favorite rpg games ever made.
Damn i love it.