Zim... because he's devoted a lot of time to murder attempts.

you got that right

zim though there are signs that they could be friends if they tried.

Gaz has this level of hatred undescribable by human and Irken alike...

ZIM!

I vote Zim, Honestly, I don't blame him, Dib is so friggin anoying! >_<

I know,sure Gaz had to deal with Dib ever since they were kids but come on,Zim is trying his ultimate best to destroy him and that takes a HUGE amount of hard work and effort and he works harder on destroying him then Gaz does.

Well their personalities differ. Zim is the kind of person that if he hated someone, he'd do everything in his means to destroy them. Gaz, if she hated someone, she wouldn't really do anything much besides little things because she's kinda lazy.

me

lol

gaz cuz shes hated him longer

PROOF:
Tried to turn Dib into bologney (however you spell it)
Tried to trap him in a room with a moose
Attemped to hypnotise him
When he needed Dib's giant head (lol) to get back to the regular world this is what happened: Dib: How do I get back? Zim: Good question! I don't really care!
Made people attack Dib (in the Christmas episode)
Theres a lot more.

Gaz is Dib's sister! I think their hatred towards each other is a brother-sister thing.

how could anyone hate dib hes soooooooooooooooooooooooooooo cut?=(