For OS X, I use the free version of Avira, however this isn't ideal as this doesn't offer zero-day protection.
I wouldn't recommend windows defender alone. I've used this alongside Kaspersky when I was with windows, often Kaspersky detected threats that Windows wouldn't. Ensure you pick software that offers "zero-day protection". That is an attack that takes advantage of a security vulnerability on the same day that the vulnerability becomes generally known. Ensure you pick anti-virus up to date.