Joffrey and Tywin seem to put up with each other for now. Tyrion would be my bet, he's been shit on by his father ever since he was born.
Cersei's a possibility, but I can't figure out a motive yet.
None of the Lannister kids are killing Tywin.
Joffrey even with his ego knows he is stronger with his grandfather around.
Cersei doesn't like getting pushed around by her father but she doesn't particularily hate him.
Tyrion hated him until the end of the last season when he asked him what did he do that was for the good of the family and Tywin admitted that it was letting him live. It's a harsh backhanded compliment but Tyrion has softened up a bit on his father for this.
Jaime? Please. Not caring about what his father thinks doesn't make him inclined to bad mouthe him let alone go to an extreme as murder.
The only Lannister with beef is Cersei's daughter shipped to Dorne and she already has a Dornish bent on revenge in King's Landing.
Unlike some characters in this show the best way to be served Tywin's comeuppance is to see his family crumble with Jaime betraying the family to ensure Sansa's safety, Cersei is successful in her machinations to not get married to Loris, Tyrion allows Sansa to escape, Joffrey dies or is exposed as a bastard and Dany coming back with Dragons and is declared the new yeezus.