Really should be in OffTopic, but Mac is the safest to use with BTC applications on it. I use linux sometimes though.
that's a myth, mac is not less secure at all, they are all about the same, the secuirty of a desktop depend mostly on the person using it
But IMHO Windows is less secure. I have you used a few OS but in that Windows seems more vulnerable to attacks than others. I haven't tested Mac though.