Riku is clearly a favourite because of his awesomeness. No matter how many times he gets knocked down he always gets back up, he's loyal to his friends & although he doesn't know it himself he is loyal to himself. Plus The Way To Dawn is one of the best Keyblades both design wise & strength wise.
My 2nd favourite is Roxas just because he has a hidden cockiness within his speech & has immense power.
Ermm well the most emotional bit for me is at the end when Sora & Kairi both read the poem together, such strong words coming from 2 stong souls
The saddest moment is also near the end, just after you beat Xemnas, Riku & Sora are on the beach & Riku tells Sora how jealous he was of him, thats true friendship...
