I'm running the free version of Bitdefender, and I like it. It's quiet, doesn't badger me, and I never have to mess with it. The only problem with the free version of Bitdefender is it's very, very basic. You can't configure very many things with it. I've not found that to be a problem, though. I've never gotten a virus while running it.
My brother runs the paid version and likes it a lot.
ETA: One thing to note, if you're running Windows 10, is that Windows Defender is actually pretty good these days. If you don't like any of the free antiviruses, you can probably get away with just running Defender so long as you aren't doing something stupid like installing random software you found on the internet.