Free: Avast // Paid: ESET (NOD32 or Smart Security). I'm gonna do a test in a few weeks. I choose those two because they're the only ones that protect well and don't drain up the resources. I'm currently using ESET Smart Security 4 Business Edition. It protects well, but I have several problems with the firewall. Also, it seems like Avast has progressed a lot.
NOD32 is the only one that doesn't come close to being good.
Way back me and afew friends use to look into Keyloggers and trojans, one in perticular was Ardamax, which is actually a well known keylogger made by ardamax software similar to the popularity of PROrat etc, basically when we tried this one on afew anti-viruses, NOD32 was the only one that didn't pick it up, naturally one of our friends who praised NOD32 wasn't with us so he didn't see the results and we hacked him with it for shits and giggles (we removed it later ofcourse).
There are many Anti-viruses out there that claim to be the best, but they rarely are.
I use Avast but i wont say it's the "best" because i know it isn't it's just what i've used for so long because it's free, easy and has silent mode.
The best choice anyone can give you is to just pick one, because at the end of the day if you're downloading something that might have a virus in it, one of 2 things will happen, either
A: the person that put it there was a kid, and is useing something known such as Ardamax, in which case any anti-virus should pick it up,
or B: there is a undetectable virus in there that is going to get you either way (Don't tell me there arn't any, there is).
Use Sandboxie if you're unsure of something, or Avast which has a Sandbox built in. If you're really un-sure and would prefer to be safe? Don't download it.