Depends on what you want you mean by Anti-Virus and what you want it to do?
Just scan email then ClamAV.
Scan user files and quarantine infections - Maldet.
If you want to go further and stop infections as they happen you'll need a commercial solution that scans all data uploaded to the server. Maldet is free and works well but doesn't stop malicious code from entering a server. Also the daily scan will just scan new files and you'll need to schedule a full scan as old files are also often hacked. A full scan is resource intensive so it's not something you can do daily on shared servers. And sites can be hacked in between full scans - so a commercial solution has a lot of merit. We use Eset Nod32. It's cheap and provides excellent protection. However it lacks any control panel integration and no end user notification. There are a few excellent anti-malware products that can integrate but they are really expensive.
There are many antivirus softwares for VPS and are FREEEE: Examples are ClamAV, Sophos, and Comodo. Best choice would be Sophos or ClamAV, they are widely Linux friendly. They are both easy to install and navigate. ClamAV can be enough for Linux but if you or clients are using multiple different devices you may want to try Sophos because this software can also prevent and remove malwares from androids, windows, and on MAC. It was the top pick free VPS antivirus in terms of performance.
They should not eat up many resources even when updates of the antivirus is necessary.
Linux Desktops are very well protected against viruses or scanners. However, to be sure of preventing powerful malwares, malicious codes, hackers, etc. to affect your system, better install one and it is free anyway. Then you are super protected
Other than this recommendations,Comodo,ClamAV,I would say Immunet.This is a free AV for Windows Server that utilizes real-time cloud analytics drawn from millions of end-users to provide virus detection. It is based on Clamwin, the Windows port of renowned open-source antivirus ClamAV.Despite its outdated interface ,this one worth's trying.
The answer should be based on your OS distro, whether you use any control panels and whether you are looking for paid or free solutions. ClamAV is a well known free scanner but you need to put your own scripts in case you need automatic scanning. But sometimes ClamAV scan can overkill the servef resources based on how many files you need to scan or how you manage the scan script. MalDet is a good one [ especially if you can integrate the ClamAV sigs from malware.expets ] and it helps you to automate the file scan with little configuration changes.
It is open source antivirus. It is used to detect the virus, malware, Trojans and malicious codes/scripts and files on the server. It is used to scan mail server too. It can be used with any platform. You just need to install the plugin for it. Updated of it are free of charge. It has command-line scanner, database scanner too.