Leasing Software

I have made a lot of studies in antivirus and computer security, and I haven’t asked this question for antivirus recommendations. I just want to know what essential things are needed in an antivirus software (basically a security software). Further, vote for:

Antivirus which:
a. Has the lease memory usage –>
b. Scans the fastest –>
c. Doesn’t miss a single malware in its virus definitions –>
d. Has the easiest user interface –>
e. As a whole is the best antivirus –>

OK guys, hope you will get it right.
Thanks for answering !!!