Assuming you are running a contemporary Microsoft operating system, nothing.

Windows XP and up include Defender. It is nothing special, but unobtrusive, free, and catches most nasties.

In addition to antivirus, ensure you make regular backups, and be careful with what unknown software you choose to run. I would also recommend creation of a non admin account for each user, particularly kids. This limits what a virus can do.

Please be specific for recommendation, not ask for best practices.

The client is willing to pay and I do not have enough time to do research.

For anyone running contemporary Microsoft operating systems, in a non managed environment, I would still recommend just configuring Defender.

The best one in my point of view is Panda free Antivirus. I've been using it for many years and never had a problem with it. I used to use Avast! free before that, but Avast! was a resource hog and in some situations it caused BSOD's. with Panda I never had such problems. It is very light on the resources it uses, stays unobtrusive if setup properly. I have never been hit with a Virus using Panda. It certainly is better than crap like Symantec (Norton) stuff.

MBAM pro - been using it for years.  It is not an AV, but rather an AM.  Use an AV as well - what ever you like.  On top put a basic free app like immunet.

Read my article on multilayered security.

https://www.experts-exchange.com/articles/18444/Multilayered-Computer-Security.html

Thanks all for the feedback.

The list of software below have been reviewed by Consumer Council of Hong Kong in April 2016 and my client
 likely to go with one of them. Please give your feedback in relation to your support experience. Thanks!

1）BullGuard Internet Security 2016；
2）G Data Internet Security 2016
3）ZoneAlarm Pro Antiviru & Firewall 2016；
4）ESET Smart Security 9
5）Kaspersy Internet Security 2016；
6）AVG Internet Security 2016
7）Avira Antivirus Pro 2016；
8）F-Secure Internet Security 2016
9）Bitdefender Internet Security 2016；
10）Trend Micro Antivirus Plus 2016
11）Norton by Symantec Norton Security Standard 2016；
12）Avast! Internet Security 2016
13）Panda Security Antivirus Pro 2016；
14）McAfee AntiVirus 2016
15）Malwarebytes Anti-Malware Premium；
16）AVG AntiVirus Free 2016
17）Avira Free Antivirus 2016；
18）Avast! Free Antivirus 2016
19）Microsoft Windows 10 - Windows Defender

Hi,

As stated above, Security greatly depends on the habit of the end user.
For strict security: Norton (Manages resources properly, easy to use, install it forget it)
For nominal security: Keep Using Windows Defender / Security Essentials (protects from majority of known viruses)
Protect yourself from internet junk and attacks: Malwarebytes

The task given is to buy antivirus software for a longer period, say three years as the client is just too busy for trying freeware.

The current antivirus software is a trial of McAfee Internet Security 2015 for a term of one year which has been expired in July 2016.

I go with Symantec and have purchased NORTON SECURITY STANDARD for a three-year licence.

Thanks all for your valuable comments.

My major concern is the ability to block links which is directing the client's computer to malicious websites and it is the basic step taken to prevent ransomware.

There are several steps to blocking redirects.  Not in any order.

Install a local alternate hosts file from MVPS.  

Install, either as part of the security suite or as an extra, a Browser Helper Object (BHO) that evaluates websites before the user lands on them.  Chrome does this well already, but you can add Web of Trust (mywot.com), which uses users' appraisals of sites as a basis for their ratings.

Block some of these websites at the perimeter.

Norton 360 and Malwarebytes Anti-Malware Premium also do a very good job identifying malicious websites  in real-time.

I'll second Paul's suggestion. Malware bytes is quite helpful. When I was a novice user, I used AdBlock Plus plugin in Mozilla for cleaner browsing experience.

MBAM is excellent - I bought several copies of the pro version when it was on sale.  There is no doubt in my mind that everyone who needs to troubleshoot computers should have MBAM as one of their tools.  That said, it should only be one of the tools you use.  As an AV for endpoints, it isn't really an AV, it's an Anti-Malware solution.  MBAM works well with your current AV as long as only one program is doing on-access scanning (I prefer MBAM do this).