Just wondering if anyone could help me.

Who was Arnolds "What u doing..." catchphrase MAINLY aimed at in the show?

His father or Willis?

This is to resolve a bet i have with a friend.

Thankyou

"What You Talkin Bout?" is the line. He is probably most famously know for saying it to Willis.

That sounds right to me, whenever I hear that line, I always think of him saying it to Willis.

Yes, it was first aimed at Willis, and then it became so popular that the writers had him saying it to everybody. On the E True Hollywood Story, one of the writers or producers said the line was originally written to read "What are you talking about Wills?"
But Gary said "Whatchoo talkin bout" and the audience loved it.