Antivirus is a bit of a religious discussion. Everyone has their Favorites (opinion) and testing depends on what you test and how you interpret the test. Note that there typically are scores for Detection, Removal, Usability obviously some offer more features than others as well. Features might be important to you IF they provide you any benefit in your usage profile. I don't use Facebook so I couldn't care less about a Facebook feature.
I think the first decision is; am I will to buy something or use a free option although you might choose best possible protection as the #1 criteria. For choices that have Free and Paid options the paid typically rates higher in testing. However, your own behavior weighs heavily on you risk factors. Your tolerance of Free nagging and ADs is another consideration. Personally I didn't like the Free software I tried because of ads and nags. if your looking for the absolute best protection then you should buy however, there's no guarantees of 100% protection.
Here's a fairly balanced review:
(Some reviews definitely seem slanted one direction or another even making top choices which seem to contradict their own test results.)
http://www.pcworld.com/article/259876/antivirus_on_windows_8_looking_at_your_options.html
It states Windows Defender scores 97% on detection and is among the top in removing what gets detected while others are better at detection and not so good at removing. Windows Defender combined with an occasional scan with the Free MalwareBytes download. You'd have a pretty good low impact unobtrusive solution unless you're in the Very High Risk Category as a user. also using a host file to block known bad sites is worth considering, as an ounce of prevention.
http://winhelp2002.mvps.org/hosts.htm This is very easy to add and just exclude the file in Defender, no need for anything more.
More Reviews:
http://anti-virus-software-review.toptenreviews.com/best-antivirus-software-for-windows-8/
http://www.pcmag.com/article2/0,2817,2372364,00.asp
I used Bitdefender last year. I didn't renew this year... can't really put my finger on why I didn't really like it, I just didn't.
I don't like what Trend has done with their products. They tried to dumb it down IMO and succeeded in hiding an obscuring what should be quite obvious.
I have several systems with Norton Internet Security or Norton 360 on them partly because most Corporations will accept that as an antivirus solution without hassles. Its also pretty easy to use with clear options yet stays out of the way mostly while just doing its job and is no longer the resource pig it used to be. On most other systems I'm mainly using the lowly Microsoft solution for its simplicity and unobtrusiveness except for some testing of other products. on pretty mush any system I will occasionally scan with another tool such a Malwarebytes just in case something was missed.
IMO, You should not feel like a slave to your antivirus solution or that its always in your face. Your system should not be unnecessarily burdened by your antivirus solution, if it is look for another option. The features should be pretty intuitive, maybe that varies by individuals, if you look at the interface and wonder what to do for even a second consider a different choice.