Personally I'm a fan of the dotted zero since it looks clean and doesn't get confused with an O or an Ø. I guess it could get confused with a theta but how often does that show up in variable names?
(Lowercase ‘θ’ is quite common to indicate a vector of parameters. I’ll admit, though, that few people are ~courageous~ adventurous enough to use the actual symbol in their variable names rather than the string ‘theta’.)