In The Outsiders, who says "I think I like it better when the old man's hitting me... At least then he knows who I am." What does it mean?

Johnny is the speaker, and he is saying that he wishes his parents paid more attention to him.

Johnny says this, because he comes from a very dysfunctional household.  Like many kids who have been abused, he just wishes his parents would pay attention to him.  Johnny’s parents seem to ignore him a lot, and when they don’t ignore him they are demeaning him or hitting him.

Johnny’s real family is the gang.  He has...

Johnny is the speaker, and he is saying that he wishes his parents paid more attention to him.

Johnny says this, because he comes from a very dysfunctional household.  Like many kids who have been abused, he just wishes his parents would pay attention to him.  Johnny’s parents seem to ignore him a lot, and when they don’t ignore him they are demeaning him or hitting him.

Johnny’s real family is the gang.  He has no problem wandering around late at night with Pony when Pony runs away after Darry hits him, because his own parents do not care where he is.   Pony and Darry fight, but Darry does not usually hit Pony.  He was frightened when Pony fell asleep and came home late, and he lost his temper.  Pony has had it with fighting with Darry, and leaves.  Johnny tells Pony that to him, being hit by his father is nothing.

Johnny is trying to make Pony feel better about his bother hitting him, by telling him at least someone cares, but he is also expressing the fact that he feels he has no family.  Pony reminds him that the gang is his family, and none of them would ever hit him.

"Shoot," I said, startled out of my misery, "you got the whole gang. Dally didn't slug you tonight 'cause you're the pet. I mean, golly, Johnny, you got the whole gang."

"It ain't the same as having your own folks care about you," Johnny said simply. (Ch. 3)

This is true.  Dally does not hit Johnny when he talks back to him, trying to get him to stop hassling the two Soc girls Cherry and Marcia.  Pony feels like if it had been anyone else talking that way to Dally, Dally would have hit him.

The entire gang is protective of Johnny.  When Pony says that Darry likes his brother Soda better and Johnny comments that at least he has people who care about him, Pony gets upset and insults Johnny’s home life.

"An' you can shut your trap, Johnny Cade, 'cause we all know you ain't wanted at home, either. And you can't blame them."

Johnny's eyes went round and he winced as though I'd belted him. Two-Bit slapped me a good one across the side of the head, and hard. (Ch. 3) 

Johnny’s home life is a sensitive issue, and not to be used against him.  Pony does have a better home life.  Johnny had not meant to make Pony mad.  Two-bit hitting Pony also shows that when anyone is mean to Johnny, the gang stands up for him.