Hi there.
Answering to the first question, its probably because of the story and gameplay. At that time there was only a very few action rpg. My first game was Chain of Memories (GBA) and tho I was stucked at Riku, and gave up the series at that time I really enjoyed it. The card system itself was interesting. Few years later just because why not, tried out KH1 and got addicted. 10 years passed since then.
My favourite Kingdom Hearts game is 2. It has a very detailed word and even better gameplay. The various suits and skills are making it really interesting to play and brings high variety to the gameplay. How they put Mickey in for a few boss and your chances to get him is also which sold me the game. The final battle, though it has a really good visual, was a disappointment. They could have done more there than that.
Favourite character, hard question. Thats either Aqua, Kairi or Xion, etc. Kairi is currently can't be used but Aqua is a really good character. Xion is just a copy of Roxas and Sora, which is shame. Roxas is only good in 358/2, he got a little chance to show of in KH2 so yeah.
Haha, that would be 10 probably.
As of now, I'm saying 8 or 9 as overall. There are ups and downs too. KH3 will decide this, or increase it probably.