well the main thing I look for in a character is that they are fun to watch. somepony like fluttershy or applejack may have more kind caring personalities but they aren't very entertaining for me. more eccentric characters like rarity and pinkie pie I usually end up liking more. they don't even have to be nice people, which is why I like diamond tiara.
my first favourite pony was definitely rarity but I do like pinkie as well. and secondary ponies like big mac and diamond tiara.