Which is grammatically correct? Or are these both equally natural?
What he is worried about most is the police coming to arrest him.
What he is most worried about is the police coming to arrest him.
Then what about these? Do they both sound natural?
What worries him most is the police coming to arrest him.
What most worries him is the police coming to arrest him.
Fine.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.