Best Antivirus for Mac?

Hey Everyone,

So I recently got a pretty good deal on a new rMBP during the Thanksgiving sales and now my first Mac is being shipped to me as we speak! I'm trying to get ready but there's one question that I can't seem to find a solid answer to. Which antivirus is the best on OS X?

I realize that a lot of people say that you don't need it, but IT at my university requires all PCs and Macs to have an antivirus installed (they can block your Internet connection if you don't). In addition, I'd feel more comfortable with one anyway. For Macs, my school recommends Avira, but I was wondering if anyone has any better ideas? Or is that a good choice? I have no prior experience with them.

By "good," I mean that I'd like something as close to MSE/Defender on the Windows side as possible (i.e. very lightweight but still with great protection). I definitely don't want something that'll bog down the computer (I still have nightmares about McAfee and Norton in the early days). I'd also prefer something free. If its any good, I do also have access to Webroot SecureAnywhere licenses for free, which also has Mac support.

Thanks in advance for any help!

Edit: I suppose I should clarify, although I'm using the term "antivirus" here, I'm really talking about a comprehensive anti malware program. Basically, something that can pick up any threats targeting OS X in addition to anything dormant on my computer that would target Windows if the file was opened there.