It depends. Some chars never make it to 50! Some do, get slotted, and then never really get played.
And then SOME are like my fire/dark/mufences/foldspace tank, who I break out anytime I want to just stomp all over everything and not die. (Ok, die once in a while, just to keep it lively... 😄 )
I don't know what makes a char a fav that I want to play, and some that are just experiments I played with longer. I have to say though, that tank is by far my favorite go to char ever, and generally I don't like tanks! She is by far the most "chill", slightly hardcore, slightly not char I have, and as a result it makes her immensely fun to play!