This is a very difficult question, as I love all my babies in different ways based on their individuality. I'm a softy with all of them.
But if I had to say which one makes my heart melt every time he comes by.....it would be Muddy. I raised him from 10 days old so I am his "momma". He comes up to me and stands on his hind legs to hug me. And he does hug me with both arms around my neck. He has such complete trust in me (more so than any other cat I've ever been owned by), that I'm awestruck at times. In the rare times that he misbehaves (and he is still all kitten), I can't discipline him cause it breaks my heart to do so - but he is so good he rarely needs it so I let it go.
But then again, there's Bogart.....my head sleeper and purr buddy for 13 years....purred me to sleep each night that entire time.....hmmmm
OK....the question is HARD! LOL