Of course a Mac.
In the whole OS X history, there has been barely any viruses or worms that are harmful to the operating system. OS X is based on unix and is mostly FreeBSD, that's why it is safer.
If you guys have any knowledge about a Mac, you'll know that you don't need a firewall nor anti-virus because most likely you'll never get one.
Someone said there aren't alot of softwares for Macs. That is incorrect. A Mac comes with tons of different software including, iSight and a tons more. The freeware list for a Mac is about the same length as for Windows. It is NOT hard to find Mac compatiable software.
Even Microsoft had its office ported to Macs. Most large open source projects are ported onto Mac anyways.
Ex:
http://b00tix.com/~pablo/
http://www.freemacware.com/
http://brilliantignorance.blogspot.com/ ... er_20.html