They all have good parts and bad parts. Some are resource hogs even though they are good, other are lightweight but don't detect everything.

I have not seen so far an antivirus that is both lightweight and that is able to detect 99% of the malware. The best name that comes in mind and which I always trust in terms of accuracy is Kaspersky, although in order to be sure, before opening any executable I always use Jotti's Virus scan or Virus Total service in order to scan the file using multiple AV's