Don't connect it to the Internet. Seriously.
If your that paranoid then no computer is safe because they are all vulnerable or allowed to remain that way intentionally.
Far as picking the best secure machine, what you have right now is as safe as it's going to get, almost no more software is required and is not advised, except for the free ClamXav to clean the Windows viruses off files you get from them, which doesn't affect a Mac. And perhaps LittleSnitch to monitor outbound behavior.
Don't install Norton, VirusBarrier, ESecurity, Sophos, MacKeeper or any "always on" antivirus as that slows down the machine or breaks it when Apple issues a update.
You can read what I've written here
And if you really want the pants scared off you, read the "Paranoid Section" here
Right out of the box, your Mac cannot be accessed remotely by anyone. Whether that becomes possible after unboxing depends on what you install, what settings you choose, etc. If you turn on anything in System Preferences -> Sharing, for example, that is a potential point of weakness that someone on the same network could exploit. (Someone on a different network would almost certainly be blocked from seeing your Mac by the network hardware.) If you install something like LogMeIn, your security is only as good as the security of your LogMeIn account.
You can find information about security on your Mac here:
They are a little outdated, not covering changes in Lion or Mountain Lion, but for the most part the Snow Leopard info is still good.