whatever fits your needs, an antivirus is only as good as its definitions,
an antivirus will only work when its switched on - most people i know switch theirs off because its always picking up virus's from limewire/bearshare/frostwire downloading - then complaining that it doesn't work,
malwarebytes is not an antivirus - its antispyware/malware, it cannot detect or remove a virus
i use rav (rising antivirus) in my windows, its free and easy to use, comes with antimalware as standard, a boot sector virus checker, auto-updates, shell scans, heuristics, rootkit checker, virus,worm and trojan remover, etc
it also has a small memory/cpu usage, even when running a scan
an all round good egg
http://www.freerav.com/
i also use it with spyware terminator
http://www.spywareterminator.com/
source - me, used them for years - and i'm a trained malware remover, so i know what i am doing