It's hard to say which KH game was the best cause they both have strength and weaknesses.
KH1: Fight Sephiroth as much as you want which is totally fun even if your ass is wooped like less than 10 seconds the first time you fight him and his music is in it to

. Has a good storyline and music, graphics well thats the problem, and it's like playing a kids game or something.
KH2: Drive system kicks butt, but hate that reaction command all you press is triangle and you automaticly do it, was hoping to push the right button to work or something. Fighting Orginisation XIII was fun especially the fighting system with the member with the cards(forgot his name and number). Sad part is that you only fight Sephiroth ONCE and thats it (unless you made a new save file just to fight him over and over again).
Both games have no difficulty, even on hard.But I did like those suprised cinematics at the end of the credits like the KH3 video

.
But the only thing I hate is that KH2 Final Mix+ is only released in Japan, and never in North America or Europe, I don't even know Japanese but I still wanna buy a copy just for the hell of it lol.
Over all I like KH2 better.