None.
I'll explain:
Antivirus is overrated. I've had more problems with them than I've ever had with viruses. Also, anything you introduce into your setup can be exploitable. Email running through antivirus? Well, then both your email program and your antivirus program are part of the possible exploit surface. Historically, antivirus do no better job than other software at avoiding being exploitable. Custom attacks that target your specifically, are unlikely to be picked up by antivirus. Same with new exploits.
There certainly are things you can do to protect yourself, mostly make sure your software is up to date, consider using Chrome, and consider buying a mac. Don't do stupid things (like opening and running ZIP-files that are mailed to you).
I ran antivirus software for probably 15 years.
How many times did it save me from an infection? Not once.
How many times did there turn out to be exploitable issues with them? Way more often.
How many times did it cause a noticeable performance degredation? More than half the time.
Bottom line; They never stopped an attack, but they made attacks easier.
I'm all for focus on computer security – it's a work-topic for me – but antivirus are basically the "alternative medicine" of the computer world, and has been for years. They were great when people were copying programs on floppies between friends and from untrusted sources. If you want to boost your security in 2016 though, focus on staying up to date, get 1Password or similar, tape your webcam, be careful with your files, use encryption, use secure chat, run through a few "how to secure X"-guides, where X is your email client and similar programs, and buy a Yubikey if you use services that supports it (google and gmail, dropbox does). And for the love of your deity, make sure you have good backups. Not just online ones, but on detached physical hardrives. One that you update regularly, one that you update once or twice a year, with just the important bits (think baby pictures etc). Keep at least one of them in a separate location. That could be what saves you from "encrypted files blackmail".
If you'd like to step it up from there, choose a decent filesystem with redundant storage that checksums, such as using FreeNAS, and also make sure you have backups on at least two types of medium (harddrives plus one of tape and optical). Step up again with ECC-ram, and streaming replication to another site.
edit to add;
The above answer targets the question "Which antivirus should I use". The answer to "Which antivirus should we use" is different. One thing is a single user, they can control their risks in other ways. In a corporate setting for example, that looks very different. You can't have all the people keep their software up to date all the time, you can't prevent all users from opening ZIP files and running programs in them, etc. Corporate settings also offers more options for where to introduce antivirus. An ingress-filter for email for example, could make good use of antivirus (clamav is often a good choice for this).
