i effing detest Riley. im sorry.

Derek has been to distant from the family lately. He hasn't bonded with John to much so I doubt it would effect John as much as if Cameron died.

Cameron is really the only one(besides Sarah), who's proved herself to John. He know's she'll protect him. If he looses that he'd feel vulnerable again. Cameron's the only one that can take those pesky terminators on.

I think John would be upset that Riley died since he believes she was his only link to the normal world but he would also realise she wasn't that important of a factor in his life, so he'd move on quickly.

All three of them "died" over the course of the show, and he seemed to be most effected by Cameron.

The answer to this is in last two episodes of the second season. He was sad, because of Derek's death, because He was part of His family.
But when John found out that John Henry took Cameron's chip, He really looked devastated and broke down.

Cameron, then Derek, but obviously Sarah is the most important to him.