It's not a necessity to have a full antivirus, unless you're sharing files a lot with Windows and Linux users.
Most AV products detect malware for those platforms as well, it's a good idea to make sure you're not unintentionally passing on a nasty to a friend or relative...
The built in XProtect system of OS X, combined with Malwarebytes Anti-Malware as a just-in-case is sufficient for most people.
Malwarebytes' mac version only goes after mac specific threats that are an actual problem, and it's a good idea to have it just in case you actually need to cleanup a threat, and aren't sure how to do it manually.
That said, if you have need for a full featured anti-virus, best to go for one with a good history on macs.
Like Intego Virusbarrier, or Sophos Anti-Virus.
Avira and Avast! have improved in the recent versions, so they should be fine too.