Malwarebytes is the name of one program, so asking what is the best one doesn't really make sense.
of the anti malware programs, of which Malwarebytes is one, then Malwarebytes is about the best about and it's totally free (you just have to make sure you click the decline button when it offers you the paid for version during the install)
The best antivirus software for free is widely debated, but to be honest the main ones mentioned on here are much the same when it comes to protection levels (MSE, Avast and AVG) and a lot of the difference in opinion is down to luck or knowing no difference.
Any of the main AV programs and a (updated) malwarebytes scan every few weeks is plenty good enough for the average user as long as they are sensible and don't just click "go ahead" if they do warn you about something.